When exporting an Org file as iCalendar and org-icalendar-timezone is
set to e.g. "Europe/Berlin", it gets inserted as "EUROPE/BERLIN" during
export (and some CalDAV servers don't like that).

This is because "%Z" gets replaced with org-icalendar-timezone, and the
FIXEDCASE parameter isn't set in the call to
replace-regexp-in-string. The attached patch fixes that.

-David