GUACAMOLE-172: Do not send potentially-invalid "sync" as a keep-alive. Use "nop" instead.

guacamole-common-js/src/main/webapp/modules/Client.js

@@ -1346,7 +1346,7 @@ Guacamole.Client = function(tunnel) {
 
         // Ping every 5 seconds (ensure connection alive)
         pingInterval = window.setInterval(function() {
-            tunnel.sendMessage("sync", currentTimestamp);
+            tunnel.sendMessage("nop");
         }, 5000);
 
         setState(STATE_WAITING);