TZOFFSET{FROM,TO}:0000 -> +0000
TZOFFSET{FROM,TO}:0000 -> +0000

Google Calendar and http://severinghaus.org/projects/icv/ complain about a TZOFFSET of 0000 but accept +0000.

Signed-off-by: Rok Garbas <91d1dd8615d34a96099572be3f37ee9127974eeb@garbas.si>

geier/icalendar,nylas/icalendar,untitaker/icalendar