Include 'linked' member for json-api collections

The options passed to the render are partitioned into adapter options
and serializer options. 'include' and 'root' are sent to the adapter,
not sure what options would go directly to serializer, but leaving this
in until I understand that better.
Define as_json instead of to_json
Merge pull request #714 from ride/to-json-as-json

Define as_json instead of to_json
Merge pull request #692 from ggordon/linked_for_jsonapi_collection

Include 'linked' member for json-api collections