Merge pull request #894 from MongoEngine/topic/not-in-style-issue

Code Cleanup
- Use not in instead of not (x in y)

mongoengine/fields.py

@@ -609,7 +609,7 @@ def to_mongo(self, document, use_db_field=True):
             return None
 
         data = document.to_mongo(use_db_field)
-        if not '_cls' in data:
+        if '_cls' not in data:
             data['_cls'] = document._class_name
         return data