/// <summary>
/// 刷新 access_token
/// </summary>
/// <param name="manager"></param>
/// <param name="oldToken">已过期的旧 access_token</param>
/// <returns></returns>
public static async Task <string> RefreshToken(this IAccessTokenManager manager, string oldToken = null)
{
var info = await manager.RefreshTokenInfo(oldToken).ConfigureAwait(false);
return(info.Token);
}