icalendar: fix issues regarding timezones without dst
* lisp/calendar/icalendar.el (icalendar--convert-tz-offset): Return
complete cons when offsets of standard time and daylight saving
time are equal.
(icalendar-export-region): Fix unbound variable warning.
* test/automated/icalendar-tests.el (icalendar--parse-vtimezone): Add
testcase where offsets of standard time and daylight saving time
are equal.
(icalendar-real-world): Fix error in test case. Expected result
was wrong when offsets of standard time and daylight saving time
were equal.