Issue with Model.create and non-embedded HasMany relationships

When you call Model.create passing in JSON representing a (non-embedded) HasMany relationship this information is never passed to the server. The attached test case (against the REST adapter) shows the problem - I ran into it while writing tests for my [new Lawnchair adapter](https://github.com/vitch/ember-model-lawnchair)...

I tracked the problem down to the `record.toJSON()` call [here](https://github.com/ebryn/ember-model/blob/master/packages/ember-model/lib/rest_adapter.js#L61) - this is where the relations are lost. I'm afraid that I can't offer a solution though