sso-auth: check for valid certificate before serving

[loganmeetsworld]

In auth/http.go/ServeHTTPS(), no check is done as to the validity of a certificate before it's used.

[mreiferson]

@loganmeetsworld I'm confused as to what this issue is referring to. At a minimum, it looks like the code in question has since been refactored. But, I also don't understand what "certificate" this issue is referring to? I don't see where either sso-{auth,proxy} accept or handle TLS certificates directly?