Spring 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

Comment From: rstoyanchev

This has been merged. Note that the JettyWebSocketSession needed the same change and I've also moved the tests to unit tests for each respective session adapter.