[BUGFIX] Correctly resolve dots inside flexform field names

When a flexform field index attribute contained a dot, only
the last value was kept.
We now correctly append the new values to the existing ones now.

Change-Id: I73e541e834c143c33ba34f7e3f3ea7a22726b074
Resolves: #79245
Releases: master, 8.7
Reviewed-on: https://review.typo3.org/51236


Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Henning Liebe <h.liebe@neusta.de>
Tested-by: Henning Liebe <h.liebe@neusta.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>