ical-recurrence: test that a date-valued RecurrenceDateTimes does not fail unfixably
Unlike DTEND, the value type of RDATE is not constrained to that of
DTSTART: RFC 5545 section 3.8.5.2 allows DATE and PERIOD as well as the
default DATE-TIME.
Combined with a DTEND, a date-valued RDATE reaches computeNewEnd with a
DateTimeStartDate as the new start and a DateTimeStartDateTime as the
original one, which is an unfixable StartStartMismatch. That error is
documented in ICal.Recurrence.Class as an internal error that should not
happen, yet it is reachable from ordinary parsed input and it halts
recurrence for the whole calendar.
Reading the added instance as an all-day one is what its value type
says. Coercing it to a time on that day would be defensible too;
failing the whole calendar is not.