fix #1590 (iCal Download storniert Buchung)

[hansmorb]

closes #1590

[codecov]

Codecov Report

Attention: Patch coverage is 4.54545% with 21 lines in your changes are missing coverage. Please review.

Project coverage is 47.49%. Comparing base (ea5a6b9) to head (2f16ffc).
Report is 1 commits behind head on master.

Files	Patch %	Lines
src/Service/iCalendar.php	0.00%	17 Missing ⚠️
src/Wordpress/CustomPostType/Booking.php	20.00%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1591      +/-   ##
============================================
- Coverage     47.54%   47.49%   -0.06%     
+ Complexity     2580     2579       -1     
============================================
  Files            95       95              
  Lines         10209    10210       +1     
============================================
- Hits           4854     4849       -5     
- Misses         5355     5361       +6     

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