calendar: Don't forget to rebuild the calendar

Don't forget to rebuild the calendar when changing the setting
'org.gnome.shell.calendar show-weekdate'. This wasn't happening anymore
and changing the setting resulted in a calendar without the days
grid.

https://bugzilla.gnome.org/show_bug.cgi?id=725533

js/ui/calendar.js

@@ -548,6 +548,7 @@ const Calendar = new Lang.Class({
     _onSettingsChange: function() {
         this._useWeekdate = this._settings.get_boolean(SHOW_WEEKDATE_KEY);
         this._buildHeader();
+        this._rebuildCalendar();
         this._update();
     },