Set 'this' context properly in FuncEventSource
Set 'this' context properly in FuncEventSource

The current code sets the `this` context when invoking the user-defined event source function. However it uses `this.calendar`, which is undefined in the `Promise` callback. It should be using `_this` to access the correct context.
kleisauke/fullcalendar,fullcalendar/fullcalendar,fullcalendar/fullcalendar,oleg-babintsev/fullcalendar,fullcalendar/fullcalendar,arshaw/fullcalendar,oleg-babintsev/fullcalendar,kleisauke/fullcalendar,kleisauke/fullcalendar,arshaw/fullcalendar,fullcalendar/fullcalendar,arshaw/fullcalendar,kleisauke/fullcalendar,oleg-babintsev/fullcalendar,arshaw/fullcalendar