[BUGFIX] ClearCacheOnLoad camelCased does not work

The em_conf setting "clearCacheOnLoad" is both used in
camelCase and lower case but only the latter one is
supported by the extension manager.

Change-Id: I637ace3bef07353b7a44c991f79d01da8e8b5547
Fixes: #60127
Releases: 6.2,6.3
Reviewed-on: https://review.typo3.org/31398
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn