[String] Fix folded in compat mode

[smnandre]

Q	A
Branch?	5.4
Bug fix?	yes
New feature?	no
Deprecations?	no
Issues	-
License	MIT

(Spotted by PHPStorm)

Both those lines of code were setting $str->string value, the second line cancelling any effect of the first.

$str->string = normalizer_normalize($str->string, $compat ? \Normalizer::NFKC : \Normalizer::NFC);
$str->string = mb_strtolower(str_replace(self::FOLD_FROM, self::FOLD_TO, $this->string), 'UTF-8');

Did not know if i had to add tests, as the $compat argument is not really defined in the interface... and seems unused in the codebase 🤷‍♂️

(It also took me a bit of time to realize the dataProvider problem 😮‍💨 )

[smnandre]

Failures unrelated

[nicolas-grekas]

Good catch! (for 5.4)

[nicolas-grekas]

Thank you @smnandre.