Invalid X-WR-TIMEZONE should not fail

[niccokunzmann]

If X-WR-TIMEZONE cannot be found, then, this should not raise an error.

X-WR-TIMEZONE:d6708488efb330c0

TODO:

• evaluate if this is the best behaviour - maybe an error is good in this case.

See also:

• do not anonymize X-WR-TIMEZONE icalendar-anonymizer#112