the DeferredMixin is going to be deprecated, as it has some very bad promise behaviours.
1. puts the thenable directly on the deferred
2. fulfils with itself, which we have plumbing to detect and correct, but steps more then 1 degree removed go undetected and easily causes horrible promise cycles. 

I would suggest mimicking the approach that the rest adapter in Ember takes when it comes to dealing with the promise.