Send presence of type 'unavailable' when logging out.

src/converse-core.js

@@ -550,6 +550,9 @@
         };
 
         this.logOut = function () {
+            if (converse.xmppstatus) {
+                converse.xmppstatus.sendPresence('unavailable');
+            }
             converse.chatboxviews.closeAllChatBoxes();
             converse.clearSession();
             if (typeof converse.connection !== 'undefined') {