JSON API adapter should return id as a string

[denkristoffer]

As per http://jsonapi.org/format/#document-structure-resource-object-ids all JSON API resource ID's must be returned as a string. Since Rails stores ID's as integers it would be nice to have this conversion happen in the adapter.

[smasry]

Refer to issue #40

[sandstrom]

Has this issue been solved?

I'm considering using Oat instead of AMS. I often checkout issues to get a feel for where the project stands.