title: update_or_create with ManyToManyField
body: <p>I have 2 models, like this:</p>
  
  <pre><code>class Subs(models.Model):
      tag = models.CharField(max_length=100)
  
  class Users(models.Model):
      name = models.CharField(max_length=100)
      tags = models.ManyToManyField(Subs)
  </code></pre>
  
  <p>I need to update tags field - add new values.
  How can I update instance of Users with update_or_create method? </p>
  
  <p>For example:</p>
  
  <pre><code>tag_1 = Subs.object.get(pk=1)
  Users.objects.update_or_create(name='test_user', tags=tag_1)
  </code></pre>
  
  <p>If user 'test_user' doesn't exist, it must be created, or if it exists tag_1 must be added to this user. </p>