Fixed #18306 -- Made deferred models issue update_fields on save

Deferred models now automatically update only the fields which are
loaded from the db (with .only() or .defer()). In addition, any field
set manually after the load is updated on save.

docs/releases/1.5.txt

@@ -42,6 +42,10 @@ keyword argument ``update_fields``. By using this argument it is possible to
 save only a select list of model's fields. This can be useful for performance
 reasons or when trying to avoid overwriting concurrent changes.
 
+Deferred instances (those loaded by .only() or .defer()) will automatically
+save just the loaded fields. If any field is set manually after load, that
+field will also get updated on save.
+
 See the :meth:`Model.save() <django.db.models.Model.save()>` documentation for
 more details.