Pong timeout firing after close?

[carlcook]

Hi,

I suspect the pong timeout handler has a minor issue.

This is the scenario (reproducible):

1. Create a web socket server
2. Specify a pong timeout handler
3. Connect a client to this server
4. Send the client a ping
5. The client disconnects (remotely) prior to the pong being sent to the server
6. Unfortunately, the pong timeout still fires, with a valid connection handle (i.e. shared_ptr.lock returns non-null, even though the client has been closed).

I'd expect for the pong timeout handler to not fire in this scenario.

If you agree that this is an issue, I'm happy enough at trying to create a PR.

Thanks,
Carl