[BUGFIX] getFolders() does not return correct keys

Resolves: #41941
Releases: 6.0
Change-Id: Ia500d4892532a7e5ec7a997f4b3b0c521d22a654
Reviewed-on: http://review.typo3.org/15858
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter
Reviewed-by: Andreas Wolf
Tested-by: Andreas Wolf