Avoid loading ActionController::TestCase in production

Ref: Shopify#3

> By referencing that object, it brings in all of the patches defined
> in `ActionController::TestCase` to the development and production
> environment, including behaviours like disabling the threading
> implementation of `ActionController::Live`

lib/active_model_serializers.rb

@@ -12,7 +12,16 @@
   ActiveSupport.on_load(:action_controller) do
     if ::ActionController::Serialization.enabled
       ActionController::Base.send(:include, ::ActionController::Serialization)
-      ActionController::TestCase.send(:include, ::ActionController::SerializationAssertions)
+
+      if ActiveSupport::VERSION::STRING < '5'
+        if defined?(Rails) && Rails.env.test?
+          ActionController::TestCase.send(:include, ::ActionController::SerializationAssertions)
+        end
+      else
+        ActiveSupport.on_load(:action_controller_test_case) do
+          ActionController::TestCase.send(:include, ::ActionController::SerializationAssertions)
+        end
+      end
     end
   end
 rescue LoadError