fix($parse) - Allow setterFn to bind correctly to promise results

When binding the result of a promise directly to the scope, getterFn
successfully extracts the data from the promise by traversing the '$$v'
attribute.

However the setterFn was not successfully traversing to the $$v
attribute, and instead writing model updates directly on the promise
itself.

The subsequent apply/digest phase of Angular was then pulling the
original value from the promise and applying it back into the viewState,
thus simulating a read-only field.

Closes issue #1827

Also see: http://stackoverflow.com/questions/16883239/angularjs-ngmodel-
field-is-readonly-when-bound-to-q-promise/16883387