Inverse async relationships are not updated

This is a failing test to describe the problem that can be seen in this jsbin http://emberjs.jsbin.com/sulup/3 (add a fixture and then remove the newly created: the room's fixtures don't get updated)

It fails because `RelationshipChangeAdd` and `RelationshipChangeRemove` expect to receive `DS.Model`s but instead get sometimes `DS.PromiseObject`s (See https://github.com/emberjs/data/blob/ff371b5d60904cff92ee4e01c1d9890ffb51d310/packages/ember-data/lib/system/changes/relationship_change.js#L364 ).