remove old nick when adding new one

JitsiConference.js

@@ -169,6 +169,9 @@ JitsiConference.prototype.removeCommand = function (name) {
  */
 JitsiConference.prototype.setDisplayName = function(name) {
     if(this.room){
+        // remove previously set nickname
+        this.room.removeFromPresence("nick");
+
         this.room.addToPresence("nick", {attributes: {xmlns: 'http://jabber.org/protocol/nick'}, value: name});
         this.room.sendPresence();
     }