Reduce implementation of Sinatra::ShowExceptions to match Rack behavior

Closes #721

Although this is a behavior change, it's one that removes confusion about the
difference between Sinatra and Rack exception handling.

As well, this change removes duplication between the two APIs and reduces
chance of bugs from upstream changes.

lib/sinatra/show_exceptions.rb

@@ -10,50 +10,12 @@ module Sinatra
   # Be careful when you use this on public-facing sites as it could reveal
   # information helpful to attackers.
   class ShowExceptions < Rack::ShowExceptions
-    @@eats_errors = Object.new
-    def @@eats_errors.flush(*) end
-    def @@eats_errors.puts(*) end
-
     def initialize(app)
-      @app      = app
-    end
-
-    def call(env)
-      @app.call(env)
-    rescue Exception => e
-      errors, env["rack.errors"] = env["rack.errors"], @@eats_errors
-
-      if prefers_plain_text?(env)
-        content_type = "text/plain"
-        exception = dump_exception(e)
-      else
-        content_type = "text/html"
-        exception = pretty(env, e)
-      end
-
-      env["rack.errors"] = errors
-
-      # Post 893a2c50 in rack/rack, the #pretty method above, implemented in
-      # Rack::ShowExceptions, returns a String instead of an array.
-      body = Array(exception)
-
-      [
-        500,
-        {
-          "Content-Type" => content_type,
-          "Content-Length" => body.join.bytesize.to_s
-        },
-        body
-      ]
+      @app = app
     end
 
     private
 
-    def prefers_plain_text?(env)
-      !(Request.new(env).preferred_type("text/plain","text/html") == "text/html") &&
-      [/curl/].index{|item| item =~ env["HTTP_USER_AGENT"]}
-    end
-
     def frame_class(frame)
       if frame.filename =~ /lib\/sinatra.*\.rb/
         "framework"

test/settings_test.rb

@@ -235,6 +235,10 @@ class SettingsTest < Minitest::Test
       klass = Sinatra.new(Sinatra::Application)
       mock_app(klass) {
         enable :show_exceptions
+        # Because this test is run under `RACK_ENV=test`, unless we change the
+        # environment or set `preferred_type` then this test will fail and
+        # `ShowExceptions` will print the exception to stdout.
+        set :preferred_type, 'text/html'
 
         get '/' do
           raise StandardError