问题:http关闭后，task估计还在运行，断点打不到，
异常信息：System.Net.HttpListenerException (0x80004005): 由于线程退出或应用
程序请求，已中止 I/O 操作。在 System.Net.HttpListener.GetContext()
代码：
方法调用
  static void Main(string[]args){
      HttpServer.Start();
      Thread.Sleep(2000);
      HttpServer.Stop();
      Console.ReadKey();
}
public class HttpServer{
    private static HttpListener httpListener;
    private static CancellationTokenSource cancelToken;
    public static void Start(){
     httpListener = new HttpListener();
            httpListener.Prefixes.Add("http://+:8048/");
            httpListener.Start();
            cancelToken = new CancellationTokenSource();
            Task.Run(() => { handlehttp(); }, cancelToken.Token);
    }


    public static void Stop(){
       httpListener.Stop();
       httpListener.Close();
       cancelToken.Cancel();
    }
    private void handlehttp(){
      while(true){
       try{
         if (cancelToken.IsCancellationRequested)
         {
          break;
         }
         if(!httpListener.IsListening){break;}
        var context = httpListener.GetContext();//结束这句一直报错
       }catch(Exception ex){
       }
     }
    }

}