fix(uws): discard any write to an aborted uWS response (#682)

This bug only exists for polling transport connections running on top
of uWS.

If the remote client abruptly disconnects (thus aborting the request)
while the server is waiting on an asynchronous operation such as
compression, the server may attempt to write a response via the aborted
response object. This causes an uncaught exception to be thrown.