ingress-nginx auth-url protection bypass

A security issue was discovered in ingress-nginx where the protection afforded by the `auth-url` Ingress annotation may not be effective in the presence of a specific misconfiguration.

If the ingress-nginx controller is configured with a default custom-errors configuration that includes HTTP errors 401 or 403, and if the configured default custom-errors backend is defective and fails to respect the X-Code HTTP header, then an Ingress with the `auth-url` annotation may be accessed even when authentication fails.

Note that the built-in custom-errors backend works correctly. To trigger this issue requires an administrator to specifically configure ingress-nginx with a broken external component.