使用HttpWebRequest請求https鏈接時，無法訪問的問題，設置ServicePointManager.SecurityProtocol安全協議

//請求前設置一下使用的安全協議類型 System.Net
if (url.StartsWith("https", StringComparison.OrdinalIgnoreCase))
{
    ServicePointManager.ServerCertificateValidationCallback = new RemoteCertificateValidationCallback((object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors errors) =>
    {
       return true; //總是接受  
    });
    ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3 | SecurityProtocolType.Tls12 | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls;
}