Add default_content_type setting

Historically, Sinatra::Response defaults to a text/html Content-Type.
However, in practice, Sinatra immediately clears this attribute after
instantiating a new Sinatra::Response object, so this default is some-
what suspect. Let's break this behavior and have Sinatra::Response
be Content-Type-less by default, and update the tests to reflect this.

Next, let's introduce a new default_content_type setting that will be
applied (instead of text/html) if the Content-Type is not set before the
response is finalized. This will be great for e.g. JSON API developers.
Let's also make it nil-able to indicate that a default Content-Type
should never be applied.

Wherever Sinatra is emitting HTML, e.g. in error pages, force the
Content-Type to text/html.

Finally, clean up the error-handling logic to behave as follows:

* Set the X-Cascade header as early as possible.
* If an error block matches and returns a value, stop processing and
  return that value.
* If we have a not found or bad request error, inspect the exception (if
  any) for an error message and present it as the response body if it
  exists, or present a default error message.

The remaining logic is the same otherwise. This should make error
handlers simpler to write and behave more consistently by not polluting
the body with a default message when none may be necessary.

README.md

@@ -2264,6 +2264,15 @@ set :protection, :session => true
       used for built-in server.
     </dd>
 
+  <dt>default_content_type</dt>
+  <dd>
+    Content-Type to assume if unknown (defaults to <tt>"text/html"</tt>). Set
+    to <tt>nil</tt> to not set a default Content-Type on every response; when
+    configured so, you must set the Content-Type manually when emitting content
+    or the user-agent will have to sniff it (or, if <tt>nosniff</tt> is enabled
+    in Rack::Protection::XSSHeader, assume <tt>application/octet-stream</tt>).
+  </dd>
+
   <dt>default_encoding</dt>
     <dd>Encoding to assume if unknown (defaults to <tt>"utf-8"</tt>).</dd>
 

lib/sinatra/base.rb

@@ -159,10 +159,6 @@ module Sinatra
   # http://rubydoc.info/github/rack/rack/master/Rack/Response/Helpers
   class Response < Rack::Response
     DROP_BODY_RESPONSES = [204, 304]
-    def initialize(*)
-      super
-      headers['Content-Type'] ||= 'text/html'
-    end
 
     def body=(value)
       value = value.body while Rack::Response === value
@@ -176,6 +172,8 @@ module Sinatra
     def finish
       result = body
 
+      headers.delete "Content-Type" if headers["Content-Type"].nil?
+
       if drop_content_info?
         headers.delete "Content-Length"
         headers.delete "Content-Type"
@@ -189,7 +187,7 @@ module Sinatra
       if calculate_content_length?
         # if some other code has already set Content-Length, don't muck with it
         # currently, this would be the static file-handler
-        headers["Content-Length"] = body.inject(0) { |l, p| l + p.bytesize }.to_s
+        headers["Content-Length"] = body.map(&:bytesize).reduce(0, :+).to_s
       end
 
       [status.to_i, headers, result]
@@ -940,15 +938,14 @@ module Sinatra
       @response = Response.new
       template_cache.clear if settings.reload_templates
 
-      @response['Content-Type'] = nil
       invoke { dispatch! }
       invoke { error_block!(response.status) } unless @env['sinatra.error']
 
       unless @response['Content-Type']
-        if Array === body and body[0].respond_to? :content_type
+        if Array === body && body[0].respond_to?(:content_type)
           content_type body[0].content_type
-        else
+        elsif default = settings.default_content_type
-          content_type :html
+          content_type default
         end
       end
 
@@ -1089,10 +1086,10 @@ module Sinatra
     # Attempt to serve static files from public directory. Throws :halt when
     # a matching file is found, returns nil otherwise.
     def static!(options = {})
-      return if (public_dir = settings.public_folder).nil?      
+      return if (public_dir = settings.public_folder).nil?
       path = "#{public_dir}#{URI_INSTANCE.unescape(request.path_info)}"
       return unless valid_path?(path)
-      
+
       path = File.expand_path(path)
       return unless File.file?(path)
 
@@ -1160,19 +1157,27 @@ module Sinatra
 
       status(500) unless status.between? 400, 599
 
-      boom_message = boom.message if boom.message && boom.message != boom.class.name
       if server_error?
         dump_errors! boom if settings.dump_errors?
         raise boom if settings.show_exceptions? and settings.show_exceptions != :after_handler
       elsif not_found?
         headers['X-Cascade'] = 'pass' if settings.x_cascade?
-        body boom_message || '<h1>Not Found</h1>'
-      elsif bad_request?
-        body boom_message || '<h1>Bad Request</h1>'
       end
 
-      res = error_block!(boom.class, boom) || error_block!(status, boom)
+      if res = error_block!(boom.class, boom) || error_block!(status, boom)
-      return res if res or not server_error?
+        return res
+      end
+
+      if not_found? || bad_request?
+        if boom.message && boom.message != boom.class.name
+          body boom.message
+        else
+          content_type 'text/html'
+          body '<h1>' + (not_found? ? 'Not Found' : 'Bad Request') + '</h1>'
+        end
+      end
+
+      return unless server_error?
       raise boom if settings.raise_errors? or settings.show_exceptions?
       error_block! Exception, boom
     end
@@ -1813,6 +1818,7 @@ module Sinatra
     set :add_charset, %w[javascript xml xhtml+xml].map { |t| "application/#{t}" }
     settings.add_charset << /^text\//
     set :mustermann_opts, {}
+    set :default_content_type, 'text/html'
 
     # explicitly generating a session secret eagerly to play nice with preforking
     begin

test/response_test.rb

@@ -3,7 +3,7 @@
 require File.expand_path('../helper', __FILE__)
 
 class ResponseTest < Minitest::Test
-  setup { @response = Sinatra::Response.new }
+  setup { @response = Sinatra::Response.new([], 200, { 'Content-Type' => 'text/html' }) }
 
   def assert_same_body(a, b)
     assert_equal a.to_enum(:each).to_a, b.to_enum(:each).to_a

test/settings_test.rb

@@ -163,6 +163,41 @@ class SettingsTest < Minitest::Test
     assert_equal :foo, @base.settings.environment
   end
 
+  describe 'default_content_type' do
+    it 'defaults to html' do
+      assert_equal 'text/html', @base.default_content_type
+    end
+
+    it 'can be changed' do
+      @base.set :default_content_type, 'application/json'
+      @base.get('/') { '{"a":1}' }
+      @app = @base
+      get '/'
+      assert_equal 200, status
+      assert_equal 'application/json', response.content_type
+    end
+
+    it 'can be disabled' do
+      @base.set :default_content_type, nil
+      @base.error(404) { "" }
+      @app = @base
+      get '/'
+      assert_equal 404, status
+      assert_nil response.content_type
+      assert_empty body
+    end
+
+    it 'may emit content without a content-type (to be sniffed)' do
+      @base.set :default_content_type, nil
+      @base.get('/') { raise Sinatra::BadRequest, "This is a drill" }
+      @app = @base
+      get '/'
+      assert_equal 400, status
+      assert_nil response.content_type
+      assert_body "This is a drill"
+    end
+  end
+
   describe 'methodoverride' do
     it 'is disabled on Base' do
       assert ! @base.method_override?