fix serializeHasMany

Now `serializeHasMany` respects `keyForRelationship` if defined.

But `serializeHasMany` still doesn't behave as expected (see https://github.com/emberjs/data/pull/1751/files#r13056066, https://github.com/emberjs/data/pull/1969 and https://github.com/emberjs/data/issues/1974).
The problem is that ember-data `v1.0.0-beta.8` doesn't include the extraction of `EmbeddedRecordsMixin` it's only in master.

@igorT can we expect a beta release with the changes?

I'll open a new PR when we have a `v1.0.0-beta.9`