rd param allows other domain

[icchy]

rd param at /oauth2/start currently allows only the path starts with /.
Otherwise the request is redirected to / even if rd param is required to allow other domain URI.
This commit will work on following nginx snippet:

location /secret {
    satisfy any;
    auth_request /oauth2/auth;
    error_page 401 = /oauth2/start?rd=$scheme://$host$uri;
}

which is useful for multiple path required to authenticated.

[icchy]

seems to be conflicted with #464 and #544