[BUGFIX] Fix checking config.metaCharset with uppercase letters

Releases: master, 8.7
Resolves: #81634
Change-Id: I48bc1885aec47a4dcc66bf556cd69d50a7781fd5
Reviewed-on: https://review.typo3.org/53291


Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Stephan Großberndt <stephan@grossberndt.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>