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

[wasifhossain]

Purpose

This PR will affect all #content_type calls in the following files:

active_model_serializers/test/action_controller/serialization_test.rb

Line 158 in 55a6b23

assert_equal 'application/json', @response.content_type

active_model_serializers/test/action_controller/explicit_serializer_test.rb

Line 74 in 55a6b23

assert_equal 'application/json', @response.content_type

Background

Rails 6 changes the default behavior of ActionDispatch::Response#content_type, which now returns the Content-Type header as it is.

To retain old behavior, i.e. retrieve the mime type only, we can either set the following configuration in application settings:

config.action_dispatch.return_only_media_type_on_content_type = true

or set it inline as:

ActionDispatch::Response.return_only_media_type_on_content_type = true

or call #media_type instead of #content_type.

Additional helpful information

Rails 6.0 ACTIONPACK release notes:
https://github.com/rails/rails/blob/6-0-stable/actionpack/CHANGELOG.md

rails/rails#36490
rails/rails#36946

[wasifhossain]

@bf4 I would like to have your thoughts on this. Not sure if this is the best way to handle the case.