Update existing shortlink registration

        Arguments:
            shortlink_id:
                Shortlink id assigned by mCASH

def update_shortlink(self, shortlink_id, callback_uri=None,
                         description=None):
        """Update existing shortlink registration

        Arguments:
            shortlink_id:
                Shortlink id assigned by mCASH
        """
        arguments = {'callback_uri': callback_uri,
                     'description': description}
        return self.do_req('PUT',
                           self.merchant_api_base_url + '/shortlink/' +
                           shortlink_id + '/', arguments)