Break up serializer to serializable_hash mapping

[callumj]

This pull request breaks up the mapping of serializer class instantiation to serializable_hash / as_json. One method maps the serializer instances and the original serializable_array emits them as serializable hashes.

This is for those that wish to extend ActiveModel::ArraySerializer but only want to modify specific behaviour such as serializable_array but wish to take advantage of the helpful serializer instantiation.

[coveralls]

Coverage increased (+0%) when pulling 2966fe3 on callumj:master into 8ac4bf9 on rails-api:master.

[spastorino]

This is already fixed in master after the rewrite. Thanks for doing the work anyway.