fix(XmppConnection): sendUnavailableBeacon

Copy/paste mistake - there's no such thing as this.connection in
the XmppConnection. Hard to track down because no error is logged.

modules/xmpp/XmppConnection.js

      * @returns {boolean} - true if the beacon was sent.
      */
     sendUnavailableBeacon() {
-        if (!navigator.sendBeacon || this.connection.disconnecting || !this.connection.connected) {
+        if (!navigator.sendBeacon || this._stropheConn.disconnecting || !this._stropheConn.connected) {
             return false;
         }