Removing attendee from scheduled recurring event should generate an EXDATE property

[KristofferFM]

In the (special) case where an organizer creates a recurrent event with only one attendee and removes that attendee from one of the instances, an iTip message with an EXDATE property should be created. Currently, the EXDATE is not correctly inserted in the iTip message, this pull request fixes that.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.78%. Comparing base (96c3152) to head (4bf2c58).
⚠️ Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #728   +/-   ##
=========================================
  Coverage     98.78%   98.78%           
  Complexity     1877     1877           
=========================================
  Files            71       71           
  Lines          5274     5274           
=========================================
  Hits           5210     5210           
  Misses           64       64           

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