TestServer.CreateClient : the HttpClient created didn't support well the cancellation.

Hello,

I just found a nasty bug on HttpClient created from TestServer.
When I call a request which took 2 seconds if I cancel it after one second.
The client created by test server didn't throw an OperationCancelledException.

Here the code with real httpClient :

```
var httpClient = new HttpClient();
using (var cts = new CancellationTokenSource(TimeSpan.FromSeconds(1)))
{
var request = new HttpRequestMessage
{
Method = HttpMethod.Get,
RequestUri = new Uri("http://localhost:4242/TimeoutTest/Action2Secs")
};
await httpClient.SendAsync(request, HttpCompletionOption.ResponseContentRead, cts.Token);
}
```

The same code with client created by the class TestServer.

```
var httpClient = new testServer.CreateClient();
using (var cts = new CancellationTokenSource(TimeSpan.FromSeconds(1)))
{
var request = new HttpRequestMessage
{
Method = HttpMethod.Get,
RequestUri = new Uri("http://localhost:4242/TimeoutTest/Action2Secs")
};
await httpClient.SendAsync(request, HttpCompletionOption.ResponseContentRead, cts.Token);
}
```