No need to extend ActiveSupport::Extend here

actionpack/lib/action_dispatch/testing/assertions/response.rb

@@ -4,8 +4,6 @@ module ActionDispatch
   module Assertions
     # A small suite of assertions that test responses from \Rails applications.
     module ResponseAssertions
-      extend ActiveSupport::Concern
-
       # Asserts that the response is one of the following types:
       #
       # * <tt>:success</tt>   - Status code was in the 200-299 range