collective/icalendar — pull request #149 by iHiroakiKawai (2014-10-20 09:09:38+00:00)
Title: duration may have extra '+' in value

from rfc5545 spec:
dur-value = (["+"] / "-") "P" (dur-date / dur-time / dur-week)

Signed-off-by: Hiroaki KAWAI