Push a notification to a specific pushed user by Pushed ID.

        Param: content -> content of Pushed notification message
               pushed_id -> user's pushed ID
               content_url (optional) -> enrich message with URL
        Returns Shipment ID as string

def push_pushed_id(self, content, pushed_id, content_url=None):
        '''Push a notification to a specific pushed user by Pushed ID.

        Param: content -> content of Pushed notification message
               pushed_id -> user's pushed ID
               content_url (optional) -> enrich message with URL
        Returns Shipment ID as string
        '''
        parameters = {
            'app_key': self.app_key,
            'app_secret': self.app_secret,
            'pushed_id': pushed_id,
            'target_alias': 'Nothing'   # Required, but seems unused
        }
        return self._push(content, 'pushed_id', parameters, content_url)