Correctly parse all types of iCal durations

This was supposed to be fixed in 6ca2535 (ical.c: Simplify and fix
ical_durtime2long(), 2014-07-28) but some cases were not covered.

Reported-by: Håkan Jerning <jerning@home.se>
Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

test/Makefile.am

@@ -46,6 +46,7 @@ TESTS = \
 	ical-002.sh \
 	ical-003.sh \
 	ical-004.sh \
+	ical-006.sh \
 	next-001.sh \
 	search-001.sh \
 	bug-002.sh \
@@ -107,4 +108,5 @@ EXTRA_DIST = \
 	data/ical-002.ical \
 	data/ical-003.ical \
 	data/ical-004.ical \
+	data/ical-006.ical \
 	data/todo