Allow http-only, to better support mock CAS servers

[donquixote]

For local development I am using a mock server for CAS.
It is much easier to set this up with http instead of https.

However, user_cas and phpCAS assume that the CAS server always uses https.

For development purposes it would be nice to allow http-only.

And I want to point out there is a difference of http protocol for the CAS server, vs for Nextcloud.

Btw, I considered to open this issue in https://github.com/apereo/phpCAS.
But we are not even using the latest version of that, so I couldn't tell if this is not already working in latest phpCAS.