title: Django : get_or_create Raises duplicate entry with together_unique
body: <p>model example</p>
  
  <pre><code>class Example(Stat):
      numeric = models.IntegerField(...) 
      date = models.DateField( auto_now_add=True,...) #auto_now_add=True was the problem
  
      class Meta:
         unique_together = ('numeric','date')
  </code></pre>
  
  <p>)</p>
  
  <p>If 72 and '2011-08-07' is already stored</p>
  
  <pre><code>Example.object.get_or_create(numeric=72,date='2011-08-07')
  </code></pre>
  
  <p>raises </p>
  
  <pre><code>django.db.utils.IntegrityError: (1062, "Duplicate entry '72-2011-08-07'
  </code></pre>
  
  <p>the question is why <code>get_or_create</code> raises the <code>IntegrityError</code>, thats the idea of using 
  <code>get_or_create</code>.</p>
  
  <p>Not sure if this is a bug, I opened a ticket <a href="https://code.djangoproject.com/ticket/16587" rel="noreferrer">https://code.djangoproject.com/ticket/16587</a></p>