allowing to pass an has many relationship on create record. Fixes #1497

Fixes #1497
cc/ @igorT 
Hum, actually when I read that: https://github.com/emberjs/data/blob/master/packages/ember-data/lib/system/model/model.js#L742, I thought it was exactly what we were looking for. So I was using preloadData for relationships only to use that path. (Also, it seemed to me that makes createRecord support passing relationships as ids as well).