deleteRecord, rollback, commit issues

ember-data has a bug where deleting records and rolling back is not working as it should. 

Suppose there is Post model that `hasMany` Comments. Also suppose we have a post with 2 comments. When deleting comment1 and rolling back, things work fine. But then deleting the comment1 again and then committing doesn't seem to update the hasMany association.

The association then gets in a bad state such that trying to delete comment2 and rolling back doesn't work like it should as comment2 stays in a dirty state.

This pull request contains a failing test that exposes this issue.