Fix events with 2 'dtstart' for all-day events in ics generation

12:43  [C4ptainCrunch] Bram: #SKANDALE ! ton ics généré par hackeragenda est pas valide !
12:49  [C4ptainCrunch] Bram: t'as parfois 2 DTSTART : https://github.com/Psycojoker/hackeragenda/blob/master/events/ics.py#L18
12:49  [C4ptainCrunch] ce qui est interdit
12:50 [C4ptainCrunch] Bram: http://www.ietf.org/rfc/rfc2445 section 4.6.1