Bring back assert_serializer for controller testing

The `assert_serializer` test helper was added in 0.9.0.apha1[1],
and was not included in 0.10.

This patch brings back the `assert_serializer` test helper. This is the last
revision[2] that has the helper. The original helper was used as base.

[1]: https://github.com/rails-api/active_model_serializers/pull/596
[2]: 610aeb2e92

- Create the AssertSerializer

- Use the Test namespace

- Make the tests pass on the Rails master
  - Rails 5 does not include `assert_template` but we need this on the tests of
the helper.
  - This add the `rails-controller-testing` to keep support on `assert_template`.

- Only load test helpers in the test environment

lib/active_model/serializer/railtie.rb

@@ -19,5 +19,9 @@ module ActiveModel
       app.load_generators
       require 'generators/serializer/resource_override'
     end
+
+    if Rails.env.test?
+      ActionController::TestCase.send(:include, ActiveModelSerializers::Test::Serializer)
+    end
   end
 end