fix($compile): reference local in isolate scope

This was really corner case:
Watcher needs to return changed value, to notify that model might have changed and one more $digest cycle needs to be performed.

The watcher, that takes care of reference binding into an isolate scope ("="), did not return changed value, if the change was from the isolate scope to the parent.

If any other watcher returned change, it worked fine, as this change caused re-digest.

Closes #1272