Serializer from block or class update

It's useful for serializer_from_block_or_class to return the serializer object and that's what the name implies it is retuning.

This then allows the json_api to access the root_name element on the child entities thus no longer requiring the root element to appear first in the hash. Relying on the order of keys in the hash should typically be avoided. (Especially on ruby 1.8)