Fix #1492 and fall back to serializing records into IDs when not embedded.

In `serializeHasMany` the JSONSerializer was trying to serialize `manyToNone` relations (don't even know what that means) instead of `manyToOne`. I found @hg-schneider had already fixed that with a pull request pending (https://github.com/emberjs/data/pull/1488). I used it as a base for my changes.

I had to modify other AMS integration tests to add empty arrays since `hasMany` is now serialized even when empty. Please review and confirm that this is correct and expected logic.