Use proxies' capabilities to rewrite URLs

[Acconut]

We currently instruct users who operate tusd behind a proxy to setup the proxy to include the original host and scheme in the forwarded requests (https://tus.github.io/tusd/getting-started/configuration/#proxies), so that tusd can generate absolute URLs that include the hostname of the proxy. These URLs are then sent in the Location headers in responses.

However, some proxies automatically rewrite the Location header to match the proxy' address. For example, Nginx does this by default through the proxy_redirect directive: http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_redirect

We should look into whether other proxies (HAProxy, Apache, Caddy etc) offer similar capabilities and if we should update our recommendation on forwarding proxy information. Maybe this is not necessary anymore.