* Merge branch 'master' of C:\Projects\PHPOffice\PHPSpreadsheet\develop with conflicts.
* Separate out date/time tests into individual tests
* Need to update the version of phpunit at some point to deal with the new assertions and deprecated assertions
* Appease the CS Gods
* More refactoring of Date/Time tests
* Replace self assertions with instance assertions (looking forward to upgrading phpunit)
* Finish refactoring of date/time tests as individual tests
* Test for DateTimeInterface rather than for DateTime
* A few strict comparisons
* Fix to test names
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