Change signal sending to include nvp_obj as sender

Since this [rather old Django commit](https://github.com/django/django/commit/704ee33f503c96b96c2682b946a11b3b42318ba7#diff-de999011ea114e4e4a35b1e412bfa0adR178) `Signal.send` looks for receivers in a cache. If we do not pass a sender explicitly, this method tries to find receivers using the first param as a dict key, raising `TypeError` (unhashable type).

Change the `send` call to include `nvp_obj` as sender and pass `params` as kwargs.