Check for string-type parameter VALUE

[martijnvankekem]

See: #635

[staabm]

please add a test case

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a5df355) 98.76% compared to head (3cf6669) 98.76%.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #636   +/-   ##
=========================================
  Coverage     98.76%   98.76%           
- Complexity     1866     1867    +1     
=========================================
  Files            71       71           
  Lines          5325     5325           
=========================================
  Hits           5259     5259           
  Misses           66       66           

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[phil-davis]

@martijnvankekem your PR comes directly from the master branch of your fork, which is a bit more diffcult to manage.
I rebased in a branch in my fork and made and up-to-date PR #640
The CI all passes there - good, no existing test cases were broken.

Please let us know the details of an example iCal structure - what does it pass in $parameters["VALUE"]
It will be good to add a unite test case that has a reasonably realistic example of this.