Changes according to feedback
Merge branch 'convert_keys' of https://github.com/mhenrixon/active_model_serializers into convert_keys

Conflicts:
	lib/active_model/convertable.rb
	lib/active_model/serializable.rb
	lib/active_model/serializer.rb
	test/fixtures/poro.rb
	test/unit/active_model/serializer/convertable_test.rb