[BUGFIX] Support string AJAX request payloads

POST, PUT and DELETE requests sent by the AJAX API now also support
string payloads, e.g. by using `JSON.stringify()`.

Resolves: #90152
Releases: master
Change-Id: I4e5fa49a4ede2388bdfdc777a5cee92a6a94c3ee
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62996


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Rodger Rüdiger <spike-shape@protonmail.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Rodger Rüdiger <spike-shape@protonmail.com>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>