Bug 11219: (follow-up) fetch only URL parameters

Bug 11219: (follow-up) fetch only URL parameters

This followup corrects the fact that when using $query->url(), both
GET and POST params are get.
Using $query->url_param() will only get params directly in URL.

Test plan :
- Enable CAS
- Go to login page : cgi-bin/koha/opac-user.pl
- Try to connect with local login using random login and password
  (they will be transmitted by POST)
- You stay to login page
- Look at CAS login URL
=> Without this patch it will contain the random login and password
   as parameters of opac-user.pl
=> With this patch it does not contain any parameter

Signed-off-by: Matthias Meusburger <matthias.meusburger@biblibre.com>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>
(cherry picked from commit b57d98517c5a9572a54412f7f4e5d872acfa85c3)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>