title: django-role-permissions does not work with transactions?
body: <p>I am struggling with the following issue for a few days now.
  The application uses django-role-permissions. Unit tests do not work correctly. Here is the example code:</p>
  
  <pre><code>from django.contrib.auth.models import User
  from rolepermissions.roles import AbstractUserRole
  
  
  class FooRole(AbstractUserRole):
      available_permissions = {
          "foo": "bar"
      }
  
  
  user = User.objects.create(username="abc")
  with transaction.atomic():
      role = FooRole.assign_role_to_user(user)
  </code></pre>
  
  <p>When running the test, the following happens when trying to assign role to user:</p>
  
  <pre><code>django.contrib.auth.models.Group.DoesNotExist: Group matching query does not exist.    
  During handling of the above exception, another exception occurred:
  MySQLdb._exceptions.OperationalError: (1305, 'SAVEPOINT s4598857152_x2 does not exist')
  </code></pre>
  
  <p>Any suggestion would be highly appreciated</p>
  
  <p><strong>EDIT</strong></p>