Access to response bodies in backend modifiers fails

[alex-schneider]

Describe the bug
Access to response bodies (backend_responses.<label>.*body) in modifiers (like set_response_headers) does not work.

To Reproduce
Steps to reproduce the behavior:

1. couper version: 0 2022-02-09 dev

2. Provide your configuration file *.hcl. Remove sensitive data.

server {
  endpoint "/" {
    proxy {
      url = "https://httpbin.org/anything"

      backend {
        set_response_headers = {
          x-foo = backend_responses.default.headers.content-type # works
          x-bug = backend_responses.default.json_body.method     # does not work
        }
        custom_log_fields = {
          foo = backend_responses.default.headers.content-type # works
          bar = backend_responses.default.json_body.method     # works
        }
      }
    }
  }
}

3. Provide a curl call for reproduction: $ curl -v localhost:8080

Expected behavior
X-Bug HTTP header field with GET value in the response.

[johakoch]

It depends on where the set_response_headers is used. With

server {
  endpoint "/" {
    proxy {
      url = "https://httpbin.org/anything"
      set_response_headers = {
        x-foo = backend_responses.default.headers.content-type
        x-bug = backend_responses.default.json_body.method
      }
    }
  }
}

or

server {
  endpoint "/" {
    set_response_headers = {
      x-foo = backend_responses.default.headers.content-type
      x-bug = backend_responses.default.json_body.method
    }
    proxy {
      url = "https://httpbin.org/anything"
    }
  }
}

both headers are set.