ZK doesn't remove a desktop after not receiving pong from a browser for a ping interval

Steps to Reproduce

1. enable WebSocket
2. configure a desktop init, cleanup listener like the attached one
3. visit any zul to create a desktop
4. open chrome developer tool > Network tab > throttling, set "offline"
5. reload the current page, chrome will show "No internet" (leave the zk page to simulate not-sending rmDesktop)
6. wait for 30s, dump server heap with visualvm, check Desktop object

Current Result

• one desktop still exists
• DesktopCleanup.cleanup() is not called

Expected Result

the desktop is removed after a ping interval timeout

Debug Information

• when you change to offline, zk calls WebSocketEndPoint.close() and cancel _pingIntervalSchedule
• this bug simulates the case that a browser doesn't send rmDesktop for any reason