add redirect for correct scheme in location header

[natbusa]

Upstream relative redirect are incorrectly rewritten when the domain in https
add the following header rewrite rule:

proxy_redirect http:// $scheme://;

[natbusa]

A more general idea (untested) could be

proxy_redirect {{ item.value.backend_protocol | default('http') }}://{{ item.key }}_backend $scheme://$host;