Fixes #1641

There's no need to explicitly raise StopIteration as that's what a bare return statement does for a generator function - so yes they're the same.
As of late 2014 return is correct and raise StopIteration for ending a generator is on a depreciation schedule. See PEP 479 for full details.
https://stackoverflow.com/q/14183803/248296
https://www.python.org/dev/peps/pep-0479/

mongoengine/queryset/queryset.py

@@ -92,7 +92,7 @@ class QuerySet(BaseQuerySet):
             # Raise StopIteration if we already established there were no more
             # docs in the db cursor.
             if not self._has_more:
-                raise StopIteration
+                return
 
             # Otherwise, populate more of the cache and repeat.
             if len(self._result_cache) <= pos: