Include ActionController::Serialization when loading 'abstract_controller'

When inheriting from ActionController::Metal instead of ActionController::Base on my controllers I'm not able to use serializers properly. There is a workaround to fix that (which is to actually include this module on my controller and override a method called `url_options`) but I don't think it's the better way to fix that.

I'm not sure if it's the better way to solve this, but it worked just fine for me.