After this change, annotated message handling methods configured to use
a destination prefix (e.g. "/app") no longer have to include the prefix
in their mapping. For example if a client sends a message to "/app/foo"
the annotated methods should be mapped with @MessageMapping("/foo").

See javadoc in SockJsService for details.

Also remove ReadOnlyMultiValueMap, CollectionUtils has a method for
that already.

Minor polish to formatting and assertion messages.

org.springframework.websocket  -> org.springframework.web.socket
org.springframework.sockjs     -> org.springframework.web.socket.sockjs

Flatten .sockjs and .sockjs.server

In addition to adding the attributes, there is now mechanism for
initializing WebSocketSession instances from attributes of the
handshake request.

Allow WebSocketHandler methods to raise an exception.

By default we install ExceptionWebSocketHandlerDecorator, which logs
unhandled exceptions and closes the session. That decorator can be
extended or replaced.

Any exceptions that remain unhandled still (i.e. no exception handling
decorator), are caught in the lowest level before propagating to the
WebSocket engine or a SockJS transport handler and handled the same
way. That means default behavior is guaranteed but also fully
customizable.