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

Fixes #1497
cc/ @igorT 
Yeah, I was thinking about that, but I'm not sure it has the same result as setProperties. For example I'm not sure that https://github.com/emberjs/data/blob/master/packages/ember-data/lib/system/model/model.js#L720 will trigger properties change. You should know more than me about that.