Set the default redirect-uri hostname to be the request hostname

Also add a flag to redirect to an https version of that hostname

If your request to google_auth_proxy is the following
    GET /someurl HTTP/1.1
    Host: somedomain.com
and redirect-url is set to ""

redirect-url will be set to http://somedomain.com/oauth2/callback
  or
redirect-url will be set to https://somedomain.com/oauth2/callback
if redirect-to-https flag is set.