使用HttpWebRequest请求https链接时，无法访问的问题，设置ServicePointManager.SecurityProtocol安全协议

//请求前设置一下使用的安全协议类型 System.Net
if (url.StartsWith("https", StringComparison.OrdinalIgnoreCase)){
    ServicePointManager.ServerCertificateValidationCallback = new RemoteCertificateValidationCallback((object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors errors) =>
    {
       return true; //总是接受  
    });
    ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3 | SecurityProtocolType.Tls12 | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls;
}