Bug 20614: Use GET instead of POST for subscription renewing

Bug 20614: Use GET instead of POST for subscription renewing

It will prevent Firefox (and certainly other browsers) to block the form
resubmit:

"To display this page, Firefox must send information that will repeat
any action (such as a search or order confirmation) that was performed
earlier."

Test plan:
Renew a subscription and save

=> You should not longer see the browser warning

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>