Attempted URL that needed authentication might not be getting restored

When using the auth proxy with Kibana it doesn't seem to restore the original URL (such as a specific dashboard or search view) that was attempted with an expired session.

Start by tracing the Go auth-proxy entry point that handles an expired session and the request URL before redirecting to SAML authentication; the payload names no file or test, so locate that flow first. Done means a Kibana request for a dashboard or search view returns to its original URL after authentication, with a regression test if the repository has coverage for this path.

Written by the indexing model from the issue text.