Explicitly set serializer for associations

Adding back the ability to specify the serializer for an association. 
Ex. has_one :author, serializer: PersonSerializer
Trying to fetch ```:options``` from ```:options``` don't contribute to readability. But it's okay for me. It's just a detail, I liked the implementation :+1: 
If you want to try a different approach you could simply add ```options[:options]``` content to the ```type``` level.