staticweb drop SSL when redirecting if behind SSL-offload LB

When redirecting, if behind a load balancer doing SSL offload, staticweb does not maintain the https for the new location.

haproxy listening to 80 and 443 on SAIO:
    frontend swift
        bind 127.0.0.1:80
        use_backend swift

    frontend swift_ssl
        bind 127.0.0.1:443 ssl crt /etc/haproxy/ssl/cert.pem
        use_backend swift

    backend swift
        server swift-proxy 127.0.0.1:8080

web is a container with staticweb enabled.

Good:
$ curl -I http://localhost/v1/AUTH_test/web
HTTP/1.1 301 Moved Permanently
Content-Type: text/html; charset=UTF-8
Location: http://localhost/v1/AUTH_test/web/

Bad (Location header is http instead of https):
$ curl -kI https://localhost/v1/AUTH_test/web
HTTP/1.1 301 Moved Permanently
Content-Type: text/html; charset=UTF-8
Location: http://localhost/v1/AUTH_test/web/

Setting url_base does not fix the issue, it just invert it.
With url_base = https:

Good:
$ curl -kI https://localhost/v1/AUTH_test/web
HTTP/1.1 301 Moved Permanently
Content-Type: text/html; charset=UTF-8
Location: https://localhost/v1/AUTH_test/web/

Bad (should be http in Location header):
$ curl -I http://localhost/v1/AUTH_test/web
HTTP/1.1 301 Moved Permanently
Content-Type: text/html; charset=UTF-8
Location: https://localhost/v1/AUTH_test/web/

Staticweb must use by default the protocol of the incoming request. Only if specified by operator, the protocol should be overwritten. To detect the protocol of the incoming request, the standard way is to use the Forwarded header. Support for the "de-facto" standard X-Forwarded-Proto would be nice too.

See:
https://tools.ietf.org/html/rfc7239#section-5.4
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Forwarded