Fix format_date with "x" format for Windows

[mvorisek]

the 1st param of IntlDateFormatter::__construct is Intl locale, not system locale

on Windows and PHP 8.3 and I getting Found unconstructed IntlDateFormatter exceptions - php/php-src#12912

[alecpl]

The intention of the code was to return "date representation based on locale" (i.e. %x from strftime()). Using English US date format is just wrong.

That being said, we dropped strftime-compatible format support in 1.6. Also, the 'x' formatter does something completely different in date().

So, we have tho options:

1. remove this whole if branch
2. fix the issue properly (and use another letter?)

[mvorisek]

What about new IntlDateFormatter(null) to use intl.default_locale?

docs: https://www.php.net/manual/en/intldateformatter.create.php#refsect1-intldateformatter.create-parameters

[alecpl]

Maybe, if you set intl.default_locale in rcmail::set_user().

[alecpl]

In other words, this has to be a user locale, not system locale.

[alecpl]

The tests pass, but we should set intl.default_locale there too. I.e. in Rcmail_Rcmail::test_format_date().