Don't use server_url for 302 Found redirects

I'm using Mailpile remotely over an authenticated NGINX SSL tunnel. Mailpile is listening on localhost:33411 but I'm accessing it over https://something. If the redirect URL includes server_url, I will get redirected to localhost:33411, which I can't access remotely.

The Location header in a 302 response works just fine with relative URLs and is conform the latest HTTP 1.1 spec.