Fix timezone conversion on strings

[PrinsFrank]

When setting a default application timezone, and a display timezone:

date_default_timezone_set('UTC');
$twig->getExtension(\Twig\Extension\CoreExtension::class)->setTimezone('Europe/Paris');

Date objects get converted, but strings don't. See #4545.

For most other paths in convertDate, the dateTime object is initialized, and afterwards the timezone gets set. For strings, the timezone is passed as a constructor argument before having the timezone set again, losing its converting behaviour. This makes the convertDate method work identical for DateTime objects as strings. This different behaviour was already spotted in #3568.

Fixes #4545 #3568 #2819