Bug 12398: Fix CAS authentication validation

Bug 12398: Fix CAS authentication validation

CGI::url_param() also returns deleted parameters so we have to check
with CGI::param() too.

Signed-off-by: Matthias Meusburger <matthias.meusburger@biblibre.com>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Took a while to get it working, but I can confirm CAS login is not
working without this patch, but does with it.

Some notes:
In order for this to work you have to add http:// in front of your
OpacBaseURL.
You will also need a CAS test server and install the certificate
on your system.

Tested with CAS test server provided by Biblibre.

Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>