Configure ActionDispatch::Response#content_type behavior on Rails 6+

in order to retain old behavior,
set `config.action_dispatch.return_only_media_type_on_content_type`
to true

it can also be configured by setting
`ActionDispatch::Response.return_only_media_type_on_content_type`

test/support/rails_app.rb

@@ -9,6 +9,10 @@ module ActiveModelSerializers
       config.action_controller.perform_caching = true
       config.action_controller.cache_store = :memory_store
 
+      if Rails::VERSION::MAJOR >= 6
+        config.action_dispatch.return_only_media_type_on_content_type = true
+      end
+
       config.filter_parameters += [:password]
     end