Allow use of @SendToUser even w/o authenticated user

Before this change, subscribing to a user destination and use of
@SendToUser annotation required an authenticated user.

This change makes it possible to subscribe to a user destination from
WebSocket sessions without an authenticated user. In such cases the
destination is associated with one session only rather than with a
user (and all their sessions).

It is then also possible to send a message to a user destination
via "/user/{sessionId}/.." rather than "/user/{user}/...".

That means @SendToUser works relying on the session id of the input
message, effectively sending a reply to destination private to the
session.

A key use case for this is handling an exception with an
@MessageExceptionHandler method and sending a reply with @SendToUser.

Issue: SPR-11309

spring-websocket/src/main/java/org/springframework/web/socket/messaging/StompSubProtocolHandler.java

@@ -301,6 +301,7 @@ public class StompSubProtocolHandler implements SubProtocolHandler, ApplicationE
 			String origDestination = stompAccessor.getFirstNativeHeader(SimpMessageHeaderAccessor.ORIGINAL_DESTINATION);
 			if (origDestination != null) {
 				stompAccessor = toMutableAccessor(stompAccessor, message);
+				stompAccessor.removeNativeHeader(SimpMessageHeaderAccessor.ORIGINAL_DESTINATION);
 				stompAccessor.setDestination(origDestination);
 			}
 		}