Fix Rails 6 content_type deprecation warning.

DEPRECATION WARNING: Rails 6.1 will return Content-Type header without
    modification. If you want just the MIME type, please use `#media_type`
    instead.

test/functional/application_controller_test.rb

@@ -163,7 +163,7 @@ class ApplicationControllerTest < ActionDispatch::IntegrationTest
         get news_updates_path(format: :json)
 
         assert_response 403
-        assert_equal "application/json", response.content_type
+        assert_equal "application/json", response.media_type
         assert_equal "Access denied", response.parsed_body["message"]
       end
     end