Refresh the authentication token.

        :param str refresh_token: The refresh token to use. May be empty if
                                  retrieved with ``fetch_token``.

def refresh_token(self, **kwargs):
        """Refresh the authentication token.

        :param str refresh_token: The refresh token to use. May be empty if
                                  retrieved with ``fetch_token``.

        """
        if 'client_secret' not in kwargs:
            kwargs.update(client_secret=self.client_secret)
        if 'client_id' not in kwargs:
            kwargs.update(client_id=self.client_id)
        return self.session.refresh_token(token_url, **kwargs)