[4.5] Issue-722: Fixing problem with broker adding exdate property with floating timezone

[phil-davis]

Backport #723 to 4.5 branch.

Part of issue #722

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.65%. Comparing base (cb00630) to head (0ddd4bc).
⚠️ Report is 5 commits behind head on 4.5.

Additional details and impacted files

@@            Coverage Diff            @@
##                4.5     #740   +/-   ##
=========================================
  Coverage     98.65%   98.65%           
- Complexity     1932     1933    +1     
=========================================
  Files            71       71           
  Lines          5296     5298    +2     
=========================================
+ Hits           5225     5227    +2     
  Misses           71       71           

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[phil-davis]

ToDo:

• sort out using $version correctly in the tests, so that we do not need to edit the literal version when it changes
• sort out code that does not work with PHP 7.1 and 7.2
• sort out which changes here should also be forward-ported to master