site with the site timezone set to Africa/Abidjan throws ValueError

[gyst]

While testing #394 @davisagli found a related bug:

A Plone site with the site timezone set to Africa/Abidjan, requesting an ical export:

Traceback (innermost last):
  Module ZPublisher.WSGIPublisher, line 181, in transaction_pubevents
  Module ZPublisher.WSGIPublisher, line 391, in publish_module
  Module ZPublisher.WSGIPublisher, line 285, in publish
  Module ZPublisher.mapply, line 98, in mapply
  Module ZPublisher.WSGIPublisher, line 68, in call_object
  Module plone.app.event.ical.exporter, line 407, in __call__
  Module plone.app.event.ical.exporter, line 403, in get_ical_string
  Module zope.component.hooks, line 144, in adapter_hook
  Module plone.app.event.ical.exporter, line 165, in calendar_from_event
  Module plone.app.event.ical.exporter, line 82, in construct_icalendar
  Module icalendar.cal, line 194, in add
  Module icalendar.cal, line 145, in _encode
  Module icalendar.prop, line 852, in __init__
ValueError: Offset value MUST be a timedelta instance