title: Django queryset update method removes objects from queryset
body: <p>This is not expected behaviour:</p>
  
  <pre><code>items = Item.objects.filter(sent=False)
  
  items.count() # 3
  
  items.update(sent=True)
  
  items.count() # 0
  </code></pre>
  
  <p>How to update objects without loosing them in queryset?</p>