HttpClient访问https的时候报错：
The request was aborted: Could not create SSL TLS secure channel.
这样修改：
var url = "https://...";
if (url.ToLower().StartsWith("https"))
{
System.Net.ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3 | SecurityProtocolType.Tls | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12;
}
using (var client = new HttpClient())
{
var r = await client.GetAsync(url);
var s = await r.Content.ReadAsStringAsync();
}