3 files changed, 5 insertions, 18 deletions
diff --git a/actionpack/lib/action_dispatch/http/response.rb b/actionpack/lib/action_dispatch/http/response.rb
index 4061ea71a3..a895d1ab18 100644
--- a/actionpack/lib/action_dispatch/http/response.rb
+++ b/actionpack/lib/action_dispatch/http/response.rb
@@ -113,10 +113,10 @@ module ActionDispatch # :nodoc:
     # The underlying body, as a streamable object.
     attr_reader :stream
 
-    def initialize(status = 200, header = {}, body = [])
+    def initialize(status = 200, header = {}, body = [], default_headers: self.class.default_headers)
       super()
 
-      header = merge_default_headers(header, self.class.default_headers)
+      header = merge_default_headers(header, default_headers)
 
       self.body, self.header, self.status = body, header, status
 
@@ -308,9 +308,7 @@ module ActionDispatch # :nodoc:
     end
 
     def merge_default_headers(original, default)
-      return original unless default.respond_to?(:merge)
-
-      default.merge(original)
+      default.respond_to?(:merge) ? default.merge(original) : original
     end
 
     def build_buffer(response, body)
diff --git a/actionpack/lib/action_dispatch/testing/integration.rb b/actionpack/lib/action_dispatch/testing/integration.rb
index 9b00616968..2fe37c5bd4 100644
--- a/actionpack/lib/action_dispatch/testing/integration.rb
+++ b/actionpack/lib/action_dispatch/testing/integration.rb
@@ -369,7 +369,7 @@ module ActionDispatch
           @request_count += 1
           @request  = ActionDispatch::Request.new(session.last_request.env)
           response = _mock_session.last_response
-          @response = ActionDispatch::TestResponse.new(response.status, response.headers, response.body)
+          @response = ActionDispatch::TestResponse.from_response(response)
           @html_document = nil
           @url_options = nil
 
diff --git a/actionpack/lib/action_dispatch/testing/test_response.rb b/actionpack/lib/action_dispatch/testing/test_response.rb
index 369ea1467c..a9b88ac5fd 100644
--- a/actionpack/lib/action_dispatch/testing/test_response.rb
+++ b/actionpack/lib/action_dispatch/testing/test_response.rb
@@ -7,11 +7,7 @@ module ActionDispatch
   # See Response for more information on controller response objects.
   class TestResponse < Response
     def self.from_response(response)
-      new.tap do |resp|
-        resp.status  = response.status
-        resp.headers = response.headers
-        resp.body    = response.body
-      end
+      new response.status, response.headers, response.body, default_headers: nil
     end
 
     # Was the response successful?
@@ -25,12 +21,5 @@ module ActionDispatch
 
     # Was there a server-side error?
     alias_method :error?, :server_error?
-
-    def merge_default_headers(original, *args)
-      # Default headers are already applied, no need to merge them a second time.
-      # This makes sure that default headers, removed in controller actions, will
-      # not be reapplied to the test response.
-      original
-    end
   end
 end