title: Django pre_save signal: check if instance is created not updated, does kwargs['created'] (still) exist?
body: <p>I am using Django's pre_save signal to implement auto_now_add. There is a lot of discussion on the internet on why you should or shouldn't implement it yourself. I do not appreciate comments on this. Neither on whether I should be rewriting the save function (I have a lot of models that use auto_now_add so using signals makes sense).</p>
  
  <p>My question is:<br>
  I would like to check if the instance is created or updated. According to some sources on the internet this can be done by testing if <code>kwargs['created']</code> is True. However <code>'created'</code> does not appear in my <code>kwargs</code> even though the instance is newly created.
  I was just wondering if it has ever existed or that it has disappeared magically.
  I know I could also test if <code>kwargs['instance'].id</code> is set (this in fact works for me), but I'd like to know if kwargs['created'] still exists.</p>