Matches on open time ranges (i.e. no end date) were not properly
handled, as `end` is simply the zero time, which confuses the
`.Before()` and `.After()` logic employed here.
This commit fixes that by adding the appropriate `.IsZero()` checks and
also adds a test case.
The current behavior unfortunately broke compatibility with DAVx5, which
by default queries only events less than 90 days ago (by using an open
time range).
5b5b542f2f