close http server after WebSocketServer.close

it's necessary to do this, or your http server is still running for period of time after you call Processor.close.  Set `httpServer = null` wouldn't work. And wsServer.close wouldn't close the http server ( for more please see file `node_modules\ws\lib\WebSocketServer.js : 111`).
BTW, I have no effort to PR this problem for three times Orz.