Fix System.ObjectDisposedException when calling StopAsync()

[Socolin]

Hello

There is a crash when stopping the webjob.

I use this in tests to start/stop some services and when it crash it prevent other services to be disposed / stopped gracefully, and the test process is stuck

            var host = builder.Build();
            using (host)
            {
                await host.StartAsync();
                await Task.Delay(2_000);
                await host.StopAsync();
            }

[JatinSanghvi]

Letting the PR go through. A better fix might be possible but I cannot think of it right away. This one does the job, for example, Cancel closes the connection if StopAsync does not get called before it, and it skips (since there's nothing to be done) if StopAsync is already called.

[JatinSanghvi]

Thanks @Socolin 🎉