Don't test received cookie with POST when there are no parameters

One of our website recently configured added behind SSO doesn't like POST request that is used in Auth.pm to test obtained cookie. I added to the test code inspired by https://gitlab.cern.ch/linuxsupport/cern-get-sso-cookie/blob/master/src/WWWCERNSSOAuth.pm#L286 that use GET vs. POST depending on non-empty formdata dictionary. I have no idea if this update works with each CERN SSO web but at least for our website this update works fine (we'll probably get rid of this elog web in near future so please consider if it makes sense to include this update in normal cern-get-sso-cookie package).