Updates username created on ADD only.

def pre_save(self, model_instance, add):
        """Updates username created on ADD only."""
        value = super(UserField, self).pre_save(model_instance, add)
        if not value and not add:
            # fall back to OS user if not accessing through browser
            # better than nothing ...
            value = self.get_os_username()
            setattr(model_instance, self.attname, value)
            return value
        return value