[BUGFIX] Followup to mod.SHARED.colPos_list fix

The internal property activeColPosList is intermediately
set to an array and not converted to a comma separated list
any more, which breaks related code.

Resolves: #39967
Releases: 6.2, 6.3
Change-Id: I6f2e4710174ad5277f9de47c2d7b2cbb7a5ebb80
Reviewed-on: http://review.typo3.org/32742
Reviewed-by: Helmut Hummel <helmut.hummel@typo3.org>
Tested-by: Helmut Hummel <helmut.hummel@typo3.org>