[FIX] web_calendar: events not appearing in arabic

Fullcalendar expect as `start` and `end` property of an event object:

> When specifying Event Objects for events or eventSources, you may
> specify a string in IETF format (ex: "Wed, 18 Oct 2009 13:00:00 EST"),
> a string in ISO8601 format (ex: "2009-11-05T13:15:30Z") or a UNIX
> timestamp.

(as seen in https://fullcalendar.io/docs1/event_data/Event_Object/)

Before this commit we could for example in arabic provide an invalid
date string "٢٠١٧-٠٣-١٦ ١٦:٠٠:٠٦" instead of "2017-03-16 16:00:06".

This commit solves this by always providing fullcalendar an ISO8601 date
string.

opw-705752

addons/web_calendar/static/src/js/web_calendar.js

@@ -639,8 +639,8 @@ var CalendarView = View.extend({
             date_stop = m_start.toDate();
         }
         var r = {
-            'start': moment(date_start).format('YYYY-MM-DD HH:mm:ss'),
-            'end': moment(date_stop).format('YYYY-MM-DD HH:mm:ss'),
+            'start': moment(date_start).toISOString(),
+            'end': moment(date_stop).toISOString(),
             'title': the_title,
             'allDay': (this.fields[this.date_start].type == 'date' || (this.all_day && evt[this.all_day]) || false),
             'id': evt.id,