ical-recurrence: test that a UTC ExceptionDateTimes excludes an instance of a zoned event
The value type of EXDATE must match that of DTSTART, so this calendar
does not conform. Other implementors do emit it though, and dealing
with other implementors' failures to comply with the spec is one of this
library's goals.
Because removeExceptionDatetimes matches DateTime values structurally,
the EXDATE is discarded by a catch-all arm that does not even emit a
fixable error, so nothing tells the caller their exception did nothing.
Excluding the instance, or reporting a fixable error, are both fine;
silently keeping it is not.