Ensure that '_reference' key is used to store the reference

I've recently updated ember and noted that `DS.Model.createRecord().get('reference')` returns an object reference, which it didn't previously.

A previous PR (https://github.com/emberjs/data/pull/596) addressed this, but it seems like it may have reverted.

I've tried to write a test that captures this behaviour, to avoid regression, but I can't. Perhaps someone can chip in?