range: Handle zero-length ranges when calculating overlap

Zero length ranges, such as for events without a DTEND could be
considered to overlap a range if they were at exactly the end of the
range. The end of a range however is the first date that is supposed to
not overlap, so a zero-length range on exactly this point should not be
considered overlapping.

This fixes cases where all-day events from the next month could show up
on the same day of the current month if they did not have a DTEND.

This also adds tests for all possible cases of such zero-length events
overlapping.

Fixes GNOME/gnome-calendar#701
1 job for fix-zero-length-ranges in 6 minutes and 3 seconds