[calendaring] Bugfix: VEVENTS with no end time.

[Lennart R. <re...@gm...>]

It turns out that VEVENTS doesn't need either duration or dtend. I
made a quick fix to the Calendaring marshaller to set the end of
events to the same as the start if the vevent doesn't have a dtend.
-- 
Lennart Regebro, Colliberty
+33 661 58 14 64