Support for arbitrary response headers

Description

Onionspray has an inject_headers_upstream parameter which uses NGINX's proxy_set_header to send HTTP headers to the upstream endpoint, but it lacks a way to set arbitrary headers back in the response sent to the user.

Tasks

• Implement support for additional headers through ngx_headers_more. Do not use add_header statements due to it's documented pitfall.
• Document in the example configuration(s).

Time estimation

• Complexity: very small (0.5 day)
• Uncertainty: low (x1.1)
• Reference (adapted)