Improve decorator serialization

[haines]

• Only serialize the attributes implemented by the decorator.
• Add to_xml and as_json methods, because although we have implemented serializable_hash, the methods that use it will fall through to the model and so it will never be called.

[steveklabnik]

Seems legit. Thank you.

[thedeeno]

@haines Is it just my setup or are you noticing that json serialization is including root by default? IIRC, this is supposed to be turned off when ActiveModel::Serializers::JSON is included. Adding self.include_root_in_json = false to my decorators resolves the problem - but I shouldn't need to.