Bug 16283 - Make OPAC registration captcha case insensitive

Bug 16283 - Make OPAC registration captcha case insensitive

The OPAC registration captcha should not be case sensitive. This patch
was moved here from bug 9393.

Test Plan:
1) Enable patron self registration
2) Test the captcha by typing in the captcha in lower case. Registration
should still succeed.

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>