Fix wrong result handling of remote calls

When using dbus-glib, single return values were special-cased to
be returned verbatim rather than as array with a single element.
This is no longer true since switching to GDBus, so fix the places
where the change was overlooked.

https://bugzilla.gnome.org/show_bug.cgi?id=678852

js/ui/userMenu.js

@@ -584,7 +584,7 @@ const UserMenuButton = new Lang.Class({
         this._session.CanShutdownRemote(Lang.bind(this,
             function(result, error) {
                 if (!error) {
-                    this._haveShutdown = result;
+                    this._haveShutdown = result[0];
                     this._updateSuspendOrPowerOff();
                 }
             }));