Missing access log in some error cases

[alex-schneider]

Describe the bug
Couper does not writes an access log in error cases while fetching muxer or other data from the incoming client request.

To Reproduce
Steps to reproduce the behavior:

1. Which Couper version? Run couper version or docker run avenga/couper version

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

server "bug" {
  api {
    endpoint "/api/**" {
      proxy {
        url = "https://httpbin.org/anything"
      }
    }
  }
}

3. Provide a curl call for reproduction

curl -v 'http://localhost:8080/api' -H 'Host: foo::'

4. See an error response or error log.

Couper writes the following log line:

2021/06/30 15:57:30 http: superfluous response.WriteHeader call from github.com/avenga/couper/server/writer.(*Gzip).WriteHeader (gzip.go:68)