Failing many-to-many test

Hi @ghempton ,

I have been using your ember-data relational adapter quite a lot and it has been working well - thanks a lot for contributing this. Recently though, I have been hitting a bug, similar to what the guys are mentioning here: https://github.com/emberjs/data/pull/724#issuecomment-13516193

I have been trying to create failing tests without much success. I looked again at my code and it looks like the issues I am facing are related to a many-to-many association. Things are saved properly on the back-end, but "sometimes" Ember returns "the" bug: 
`Uncaught Error: Attempted to handle event `didCommit` on <App.User:ember1784:null> while in state rootState.loaded.saved. Called with undefined`

(I say "sometimes" because a simple POST of the nested structure works well, but it starts to fail when I combine PUTs and POSTs).

So I built a simple many-to-many test with your ember-data fork just to check, and it seems to fail - cf my pull request.

Am I missing something (I could well be, I apologize in advance :-) ), or is there really an issue?

Thanks!
PJ