Create assert_response_schema test helper

It is a common pattern to use JSON Schema to validate a API response[1], [2]
and [3].

This patch creates the `assert_response_schema` test helper that helps people do
this kind of validation easily on the controller tests.

[1]: https://robots.thoughtbot.com/validating-json-schemas-with-an-rspec-matcher
[2]: https://github.com/sharethrough/json-schema-rspec
[3]: https://github.com/rails-api/active_model_serializers/issues/1011#issuecomment-127608121

lib/active_model/serializer/configuration.rb

@@ -21,6 +21,7 @@ module ActiveModel
 
         config.adapter = :attributes
         config.jsonapi_resource_type = :plural
+        config.schema_path = 'test/support/schemas'
       end
     end
   end

lib/active_model/serializer/railtie.rb

@@ -21,6 +21,7 @@ module ActiveModel
     end
 
     if Rails.env.test?
+      ActionController::TestCase.send(:include, ActiveModelSerializers::Test::Schema)
       ActionController::TestCase.send(:include, ActiveModelSerializers::Test::Serializer)
     end
   end