title: south migration: "database backend does not accept 0 as a value for AutoField" (mysql)
body: <p>I'm new to django and trying to have a Foreign key back to users for an assignee and reporter.
  But when i'm trying to apply the change with South i get the error</p>
  
  <pre><code>ValueError: The database backend does not accept 0 as a value for AutoField.
  </code></pre>
  
  <p>My Model Code:</p>
  
  <pre><code>class Ticket(models.Model):
      title = models.CharField(max_length=80)
      text = models.TextField(blank=True)
      prioritys = models.ForeignKey(Prioritys)
      ticket_created = models.DateTimeField(auto_now_add=True)
      ticket_updated = models.DateTimeField(auto_now=True)
      assignee = models.ForeignKey(User, null=True, related_name='assignee')
      reporter = models.ForeignKey(User, null=True, related_name='reporter')
  
      def escaped_text(self):
          return markdown.markdown(self.text)
  
      def __unicode__(self):
          return self.text
  </code></pre>