title: Django: Adding "NULLS LAST" to query
body: <p>I would like to sort a model by using Postgresql's "NULLS LAST" option.
  How could it be done?</p>
  
  <p>I tried something like</p>
  
  <p><code>MyModel.objects.all().extra(order_by=('-price', 'NULLS LAST'))</code>  </p>
  
  <p>But I get </p>
  
  <p>"Cannot resolve keyword 'NULLS LAST' into field"</p>