Ember Data fixture adapter saving record loses has many relationships

@billcchkk could you open a PR against emberjs/data? I’m facing this same problem here while writing my tests.

@jefkoslowski My App.Cart hasMany App.Items. After .createRecord() is called, cart.get('items.length') is 1, but as soon as I call cart.save(), cart.get('items.length') returns 0. It’s definitely a bug in Ember Data.

Thanks.