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.

1 files changed, 12 insertions, 1 deletions

diff --git a/actionpack/lib/action_dispatch/http/response.rb b/actionpack/lib/action_dispatch/http/response.rb
index 63d8f6b585..ea3692951f 100644
--- a/actionpack/lib/action_dispatch/http/response.rb
+++ b/actionpack/lib/action_dispatch/http/response.rb
@@ -86,6 +86,7 @@ module ActionDispatch # :nodoc:
 
     cattr_accessor :default_charset, default: "utf-8"
     cattr_accessor :default_headers
+    cattr_accessor :return_only_media_type_on_content_type, default: false
 
     include Rack::Response::Helpers
     # Aliasing these off because AD::Http::Cache::Response defines them.
@@ -243,7 +244,17 @@ module ActionDispatch # :nodoc:
 
     # Content type of response.
     def content_type
-      super.presence
+      if self.class.return_only_media_type_on_content_type
+        ActiveSupport::Deprecation.warn(
+          "Rails 6.1 will return Content-Type header without modification." \
+          " If you want just the MIME type, please use `#media_type` instead."
+        )
+
+        content_type = super
+        content_type ? content_type.split(/;\s*charset=/)[0].presence : content_type
+      else
+        super.presence
+      end
     end
 
     # Media type of response.