Rails 6.1 will return Content-Type header without modification ... use `#media_type` instead

I got the same error message when I was upgrading my app from Rails 5.2.3 to Rails 6.0.0-rc1

config/application.rb

# this was the line before
# config.load_defaults 5.2
config.load_defaults 6.0

In my case I had to change the version from 5.2 to 6.0

You can make the warning go away by adding this to your application.rb:

config.action_dispatch.return_only_media_type_on_content_type = false

You would need to make sure that if you are using content_type anywhere in your code that you replace that with media_type before making that change.