Fixed unhandled exception when cancel is called on the CancellationToken

[sam1308]

Hi, first of all great work!

Code to reproduce:

var cts = new CancellationTokenSource();
var websocket = new WebSocket("ws://1.2.3.4/dummy", cts.Token);

// After any moment in time
cts.Cancel(); // this causes an uncaught TaskCancelledException

Which led me to the async void lambda expression ;)

[Xilconic]

Any update on this pull request?