Bug 19788: Preserve case sensitivity when creating Local preferences

Bug 19788: Preserve case sensitivity when creating Local preferences

Fix a bug tranforming new local preference variable name in lowercase.

TO TEST:

1. Admin > System preferences > Local use
2. Button New preference. Fill variable field with 'TESTfoo'. Click
   Save.
3. The new variable is named: 'testfoo'.
4. Apply the patch.
5. Button New preference. Fill variable field with 'TESTfoo2'. Click
   Save.
6. The new variable is properly named: 'TESTfoo2'.

Signed-off-by: Claire Gravely <claire.gravely@bsz-bw.de>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>