Make `ActionDispatch::Response#content_type` behavior configurable

I changed return value of `ActionDispatch::Response#content_type` in #36034.
But this change seems to an obstacle to upgrading. https://github.com/rails/rails/pull/36034#issuecomment-498795893

Therefore, I restored the behavior of `ActionDispatch::Response#content_type`
to 5.2 and deprecated old behavior. Also, made it possible to control the
behavior with the config.

3 files changed, 31 insertions, 0 deletions

diff --git a/railties/lib/rails/application/configuration.rb b/railties/lib/rails/application/configuration.rb
index da1c433e52..72c7ff169f 100644
--- a/railties/lib/rails/application/configuration.rb
+++ b/railties/lib/rails/application/configuration.rb
@@ -129,6 +129,7 @@ module Rails
 
           if respond_to?(:action_dispatch)
             action_dispatch.use_cookies_with_metadata = true
+            action_dispatch.return_only_media_type_on_content_type = false
           end
 
           if respond_to?(:action_mailer)
diff --git a/railties/lib/rails/generators/rails/app/templates/config/initializers/new_framework_defaults_6_0.rb.tt b/railties/lib/rails/generators/rails/app/templates/config/initializers/new_framework_defaults_6_0.rb.tt
index d25552e923..abb03e761b 100644
--- a/railties/lib/rails/generators/rails/app/templates/config/initializers/new_framework_defaults_6_0.rb.tt
+++ b/railties/lib/rails/generators/rails/app/templates/config/initializers/new_framework_defaults_6_0.rb.tt
@@ -16,6 +16,9 @@
 # It's best enabled when your entire app is migrated and stable on 6.0.
 # Rails.application.config.action_dispatch.use_cookies_with_metadata = true
 
+# Change the return value of `ActionDispatch::Response#content_type` to Content-Type header without modification.
+# Rails.application.config.action_dispatch.return_only_media_type_on_content_type = true
+
 # Return false instead of self when enqueuing is aborted from a callback.
 # Rails.application.config.active_job.return_false_on_aborted_enqueue = true
 
diff --git a/railties/test/application/configuration_test.rb b/railties/test/application/configuration_test.rb
index 6f9711cb37..f6bec3242a 100644
--- a/railties/test/application/configuration_test.rb
+++ b/railties/test/application/configuration_test.rb
@@ -2436,6 +2436,33 @@ module ApplicationTests
       assert_nil ActiveStorage.queues[:purge]
     end
 
+    test "ActionDispatch::Response.return_only_media_type_on_content_type is false by default" do
+      app "development"
+
+      assert_equal false, ActionDispatch::Response.return_only_media_type_on_content_type
+    end
+
+    test "ActionDispatch::Response.return_only_media_type_on_content_type is true in the 5.x defaults" do
+      remove_from_config '.*config\.load_defaults.*\n'
+      add_to_config 'config.load_defaults "5.2"'
+
+      app "development"
+
+      assert_equal true, ActionDispatch::Response.return_only_media_type_on_content_type
+    end
+
+    test "ActionDispatch::Response.return_only_media_type_on_content_type can be configured in the new framework defaults" do
+      remove_from_config '.*config\.load_defaults.*\n'
+
+      app_file "config/initializers/new_framework_defaults_6_0.rb", <<-RUBY
+        Rails.application.config.action_dispatch.return_only_media_type_on_content_type = true
+      RUBY
+
+      app "development"
+
+      assert_equal true, ActionDispatch::Response.return_only_media_type_on_content_type
+    end
+
     test "ActionMailbox.logger is Rails.logger by default" do
       app "development"