title: Spring Websockets @SendToUser without login?
body: <p>I have a simple spring application with websocket functionality and everything works so far.
  Now I want to send a message from my server to a specific client using the @SendToUser annotation. This gives me the error "Ignoring message, no principal info available". I understand that i have no login whatsoever on my server, so every user is "anonymous" and does not have a principal (I am not using spring security for now). But every user has a session-id. Isnt it possible to use the session id somehow to differentiate between users? How can i achieve that so my users get a principal which corresponds to the session-id?</p>