Fix iteration on querysets.

If iteration of a queryset was interrupted (by a break, or a caught
error), the next iterator would start from the second element as the
cursor had already moved to the first. This is fixed by adding a rewind
into the __iter__ method.

mongoengine/queryset.py

@@ -1342,6 +1342,7 @@ class QuerySet(object):
             raise OperationError(u'Update failed [%s]' % unicode(e))
 
     def __iter__(self):
+        self.rewind()
         return self
 
     def _sub_js_fields(self, code):