[BUGFIX] Incorrect default extension configuration

On installing an extension with an individual configuration, the
persisted information contains too much data (e.g. form label,
name, etc.) is stored as an array.

Change-Id: I87a925fc7d6f0664fc6a4302e4f9abbe6eddf5cf
Fixes: #40450
Releases: 6.0
Reviewed-on: http://review.typo3.org/14231
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader