title: Django model update or create object with unique constraint
body: <p>There is a model:</p>
  
  <pre><code>class Proxy(models.Model):
          host = models.CharField(max_length=100,)
          port = models.CharField(max_length=10,)
          login = models.CharField(max_length=100,)
          password = models.CharField(max_length=100,)
      class Meta:
          unique_together = ("host", "port")
  </code></pre>
  
  <p>I added batch of proxies in admin interface, and one of them is 0.0.0.0:0000, login=123, password=123.
  Then I add another batch of proxy, and one of them is the same 0.0.0.0:0000, but with new login=234 and password=234.
  Is any possibility to override save method of model to get behaviour like "insert ... on conflict (host, port) do update set login=login, password=password". 
  Django 2, db - Postgres.</p>