Merge pull request #1731 from van4oza/patch-1

Insert null values fix

mongoengine/document.py

@@ -1013,6 +1013,7 @@ class DynamicDocument(Document):
         field_name = args[0]
         if field_name in self._dynamic_fields:
             setattr(self, field_name, None)
+            self._dynamic_fields[field_name].null = False
         else:
             super(DynamicDocument, self).__delattr__(*args, **kwargs)