WIn7 下使用 NetHttpClient 请求HTPPS 网站

在WIN7 下使用 HttpClient 会报以下两种错 ;
1 Server Certificate Invalid or not present
2 Error sending data: (12175) 发生了安全错误.

3 System.Net.WebException: 请求被中止: 未能创建 SSL/TLS 安全通道。

经查询资料是因为WIn7 默认不支持这个协议
详细见：
https://support.microsoft.com/en-us/help/3140245/update-to-enable-tls-1-1-and-tls-1-2-as-default-secure-protocols-in-wi

解决方法：

指定 HttpClient.SecureProtocols  为TLS12

代码：

procedure TForm2.Button1Click(Sender: TObject);
var
  str:string;
  ss:TStringStream;//TStreamstring
 begin
   try
    ss:=TStringStream.Create();
    //WIN 7 下使用 需要 指定 默认的协议TLS12
   http.SecureProtocols:=[THTTPSecureProtocol.TLS12];
    http.Get(url,ss);
    memo1.Text:=ss.DataString;
   finally
      ss.Free;
   end;
end;

   private void button1_Click(object sender, EventArgs e)
        {
            //ServicePointManager.ServerCertificateValidationCallback =
            //    new System.Net.Security.RemoteCertificateValidationCallback(CheckValidationResult);
            string data;
            HttpClient http = new HttpClient();
            System.Net.ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;
            HttpResponseMessage response = http.GetAsync("https://kindao1.github.io/LimitTW/").Result;
            textBox1.AppendText(response.ToString()+"\r\n");
            var re = response.Content.ReadAsStringAsync();
            data =re.Result;
            textBox1.AppendText(data);
        }