JSONSerializer should use the attrs hash when extracting records

Currently the `attrs` hash which is used to declare a mapping between property names on `DS.Model` records and payload keys only works correctly in the `RESTSerializer`.

The `JSONSerializer` will use the `attrs` mappings when serializing records but it does **NOT** used the `attrs` hash when extracting records. This pr ensures the transformations declared in the `attrs` mapping are run in the `JSONSerializer`'s normalize function.