title: Django - auto_now=True did not work for custom actions
body: <p>My Model has the following field. It has to update whenever there is a change in that model.</p>
  
  <p>modified_at = models.DateTimeField(auto_now=True)</p>
  
  <p>and I wrote a custom action to change the status of this model in the admin interface.</p>
  
  <p>this particular field works fine for all kind of updates except the mentioned custom actions in the admin interface. Any ideas?</p>