send a close msg with userId to '/sys/close' when websockets closed

here is my use case, for every user connect I create a new synth, so when the user close the page, I want to free the synth

````
OSCdef(\close, { |args|
  var userID = args[1];
  var nodeID = ~userNodeDict.at(userID);

  s.sendMsg(\n_free, nodeID);

}, '/sys/close', nil, 9001);
````