刷新 access token

        :param refresh_token: OAuth2 refresh token
        :return: JSON 数据包

def refresh_access_token(self, refresh_token):
        """刷新 access token

        :param refresh_token: OAuth2 refresh token
        :return: JSON 数据包
        """
        res = self._get(
            'sns/oauth2/refresh_token',
            params={
                'appid': self.app_id,
                'grant_type': 'refresh_token',
                'refresh_token': refresh_token
            }
        )
        self.access_token = res['access_token']
        self.open_id = res['openid']
        self.refresh_token = res['refresh_token']
        self.expires_in = res['expires_in']
        return res