* Fix Issue 1441 (isDateTime and Formulas)
When you have a date-field which is a formula, isDateTime returns false.
https://github.com/PHPOffice/PhpSpreadsheet/issues/1441
Report makes sense; fixed as suggested. Also fixed a few minor
related issues, and added tests so that Shared/Date and Shared/TimeZone
are now completely covered.
Date/setDefaultTimeZone and TimeZone/setTimeZone were not consistent
about what to do in event of failure - return false or throw.
They will now both return false, which is what Date's function
said it would do in its doc block anyhow. Date/validateTimeZone will
continue to throw; it was protected, but was never called outside
Date, so I changed it to private.
TimeZone/getTimeZoneAdjustment checked for 'UST' when it probably
meant 'UTC', and, as it turns out, the check is not even needed.
The most serious problem was that TimeZone/validateTimeZone does not
check the backwards-compatible time zones. The timezone project
aggressively, and very controversially, "demotes" timezones;
such timezones eventually wind up in the PHP backwards-compatible list.
We want to make sure to check that list so that our applications do not
break when this happens.
Because even if it doesn't make a difference in practice, it is
technically more correct to call static methods statically. It
also better advertise that those methods can be used from any context.
