Make WebSocket fields available after it is closed

Update some native WebSocket session getters to return basic
information after it is closed. It is required for example in
SubProtocolWebSocketHandler#afterConnectionEstablished() or
StompSubProtocolHandler#afterSessionStarted().

Issue: SPR-11621
Update JettyWebSocketSession

Ensure the JettyWebSocket session can return the Principal and accepted
WebSocket sub-protocol even after the session is closed.

Issue: SPR-11621
Move tests to packages where they belong
Merge pull request #505 from sdeleuze/SPR-11621
Fix failing test