Serialize id of belongsTo relationship via serializeId

This changes the serialization of a `belongsTo` relationship in `RESTSerializer`, so the id of the referenced model of the relationship is serialized via the adapters' `serializeId` method.

`Serializer#serializeId` is already used when creating the array of ids to delete in the `RESTAdapter#deleteRecords` (see https://github.com/pangratz/data/blob/master/packages/ember-data/lib/adapters/rest_adapter.js#L227) so this change makes this behavior consistent ...