stop applying default headers in ActionDispatch::Response

I'm making this change so that I can construct response objects that
*don't* have the default headers applied.  For example, I would like to
construct a response object from the return value of a controller.

If you need to construct a response object with the default headers,
then please use the alternate constructor:
`ActionDispatch::Response.create`

actionpack/CHANGELOG.md

@@ -1,3 +1,8 @@
+*   ActionDispatch::Response#new no longer applies default headers.  If you want
+    default headers applied to the response object, then call
+    `ActionDispatch::Response.create`.  This change only impacts people who are
+    directly constructing an `ActionDispatch::Response` object.
+
 *   Accessing mime types via constants like `Mime::HTML` is deprecated.  Please
     change code like this:
 

actionpack/lib/action_controller/metal.rb

@@ -135,7 +135,7 @@ module ActionController
     end
 
     def self.make_response!(request)
-      ActionDispatch::Response.new.tap do |res|
+      ActionDispatch::Response.create.tap do |res|
         res.request = request
       end
     end

actionpack/lib/action_controller/metal/live.rb

@@ -236,6 +236,10 @@ module ActionController
         end
       end
 
+      def initialize(status = 200, header = {}, body = [])
+        super(status, Header.new(self, header), body)
+      end
+
       private
 
       def before_committed
@@ -257,10 +261,6 @@ module ActionController
         buf
       end
 
-      def merge_default_headers(original, default)
-        Header.new self, super
-      end
-
       def handle_conditional_get!
         super unless committed?
       end

actionpack/lib/action_controller/test_case.rb

@@ -585,7 +585,7 @@ module ActionController
       end
 
       def build_response(klass)
-        klass.new
+        klass.create
       end
 
       included do

actionpack/lib/action_dispatch/http/response.rb

@@ -103,13 +103,21 @@ module ActionDispatch # :nodoc:
       end
     end
 
+    def self.create(status = 200, header = {}, body = [], default_headers: self.default_headers)
+      header = merge_default_headers(header, default_headers)
+      new status, header, body
+    end
+
+    def self.merge_default_headers(original, default)
+      default.respond_to?(:merge) ? default.merge(original) : original
+    end
+
     # The underlying body, as a streamable object.
     attr_reader :stream
 
-    def initialize(status = 200, header = {}, body = [], default_headers: self.class.default_headers)
+    def initialize(status = 200, header = {}, body = [])
       super()
 
-      header = merge_default_headers(header, default_headers)
       @header = header
 
       self.body, self.status = body, status
@@ -345,10 +353,6 @@ module ActionDispatch # :nodoc:
     def before_sending
     end
 
-    def merge_default_headers(original, default)
-      default.respond_to?(:merge) ? default.merge(original) : original
-    end
-
     def build_buffer(response, body)
       Buffer.new response, body
     end

actionpack/lib/action_dispatch/testing/test_response.rb

@@ -7,7 +7,7 @@ module ActionDispatch
   # See Response for more information on controller response objects.
   class TestResponse < Response
     def self.from_response(response)
-      new response.status, response.headers, response.body, default_headers: nil
+      new response.status, response.headers, response.body
     end
 
     # Was the response successful?

actionpack/test/dispatch/response_test.rb

@@ -4,7 +4,7 @@ require 'rack/content_length'
 
 class ResponseTest < ActiveSupport::TestCase
   def setup
-    @response = ActionDispatch::Response.new
+    @response = ActionDispatch::Response.create
   end
 
   def test_can_wait_until_commit
@@ -217,7 +217,7 @@ class ResponseTest < ActiveSupport::TestCase
         'X-Content-Type-Options' => 'nosniff',
         'X-XSS-Protection' => '1;'
       }
-      resp = ActionDispatch::Response.new.tap { |response|
+      resp = ActionDispatch::Response.create.tap { |response|
         response.body = 'Hello'
       }
       resp.to_a
@@ -236,7 +236,7 @@ class ResponseTest < ActiveSupport::TestCase
       ActionDispatch::Response.default_headers = {
         'X-XX-XXXX' => 'Here is my phone number'
       }
-      resp = ActionDispatch::Response.new.tap { |response|
+      resp = ActionDispatch::Response.create.tap { |response|
         response.body = 'Hello'
       }
       resp.to_a