Support reverse proxy the NextCloud way

[Toilal]

In some network configurations involving a reverse proxy, the base url generated by the underlying SAML library is not consistent with the way it's generated in NextCloud.

For example, it may generate http:// urls instead of https:// when the SSL Layer is handled by a proxy, even when NextCloud URLGenerator#getAbsoluteURL effectively generates https:// urls.

This change setup SAML library to use the Server Protocol and Server Host as returned by the NextCloud Request object to build SAML urls properly.

[hairmare]

Safari and Firefox both seem to currently display a security warning due to http only URLs in some redirects. It looks like this PR would fix the issue.

[rullzer]

Right. It makes sense... let me see if I can do a test setup with this.

[Toilal]

I have rebased on current master. @rullzer Have you made some tests with those changes ?

[blizzz]

I am going through the open PRs and a there is a bunch of dusty ones here :sweat_smile:

Rebased this one. I understand this is still relevant.