Fixes bug using positional operator to update embedded documents.

append_field wasn't getting reset to True in the loop, so fields wouldn't
get appended to clean_fields after str was encountered

[#354]

mongoengine/queryset.py

@@ -1276,8 +1276,8 @@ class QuerySet(object):
                 parts = []
 
                 cleaned_fields = []
-                append_field = True
                 for field in fields:
+                    append_field = True
                     if isinstance(field, str):
                         # Convert the S operator to $
                         if field == 'S':