feature-check for bufferedAmount

lib/transports/websocket.js

@@ -89,7 +89,7 @@ WS.prototype.write = function(packets){
   this.socket.send(parser.encodePayload(packets));
   // check periodically if we're done sending
   this.bufferedAmountId = this.setInterval(function() {
-    if (socket.bufferedAmount == 0) {
+    if ('bufferedAmount' in this.socket && this.socket.bufferedAmount == 0) {
       clearInterval(self.bufferedAmountId);
       this.writable = true;
       this.emit('drain');