Ignore request.accept() if TCP was closed

Related issue: https://github.com/theturtle32/WebSocket-Node/issues/167
I've avoided using the `readable` and `writable` properties of streams, even though they would be *extremely* handy, because they're not included in the Node API documentation, so I have to assume they are private properties that may be subject to change without notice, and would rather not depend on them.