Ya now that I think about it, this makes sense now that i thought about it. (so ignore me comment)

As with extend prototypes turned off we do not mix in all the ember goodness to arrays. As RSVP does create a new array, we can use `Em.A` to convert the array into one with all the ember goodness. 

If the array has already been emberized, it is nearly a noop. I wonder if Ember.A itself should just fall-through if Extend prototypes is enabled.