Function upstream_url 

pub fn upstream_url(endpoint: &str, target: &str) -> String

The absolute URL a proxied request targets, given the endpoint and the request target.

A bare host is read as https, for the reason crate::session::http::ReqwestBackend gives: the platform hands back a hostname, and defaulting to plain HTTP on the strength of a missing prefix would put a bearer token on the wire in clear text.