WIP backchannel to skip assimilation when the fulfillment is a promise

TL;DR Resolving a promise with itself is a mind-blowing experience

This is merely to preserve a common ember idiom: fulfillment of a
promise-entity with itself:

```javascript
user = User.find(1);

user.then(function(user){
  //
};
```

The above example is not compatible with [The Promise Resolution Procedure](https://github.com/promises-aplus/promises-spec#the-promise-resolution-procedure)

This backchannel merely allows us to skip the assimilation step,
thereby allowing us to preserve the current behaviour and api.