JSON:API adapter not up-to-date with the spec

[georgebrock]

I was looking at using Oat for JSON:API, and found that the adapter's not following the latest version of the spec. The specific things I noticed were:

• Attributes (other than "id" and "type") should be nested under an "attributes" key.
• "id" should be a string (JSON API adapter should return id as a string #47)
• Linked entities should be under a top-level "included" key (similar to JSONAPI linked entity names. #40, but the name has changed from "linked")

It's my first time using Oat, so some of these problems could have been user error.