Don't gzip small responses

[filex]

Describe the bug

gzip is means to make a response smaller. However, there is a small overhead for gzip itself making it unsuitable for very small responses.

To Reproduce

1. Which Couper version? 0.8
2. Provide your configuration file *.hcl. Remove sensitive data.

server "gzip" {
 endpoint "/" {
  response {
    body =  ""
  }
 }
}

3. Provide a curl call for reproduction

$ curl --compressed -i http://localhost:8080/
HTTP/1.1 200 OK
Content-Encoding: gzip
Content-Type: text/plain
Server: couper.io
Vary: Accept-Encoding
Date: Wed, 07 Apr 2021 15:59:25 GMT
Content-Length: 23

Couper sends 23 bytes of body (plus 22 bytes for the Content-Encoding header).

Expected behavior

Very small responses like that should not be gzip'ed, regardless of the client's accept-encoding header.

We should use a minimal body length before choosing gzip. 60 bytes could be a good start. If the response to be send is shorter, don't gzip, don't set Content-Encoding: gzip and don't add accept-encoding to the Vary list.

[alex-schneider]

Done with #256