Request with referenced backend fails

[malud]

Describe the bug
The request backend configuration seems to fail if the backend string reference is used.

To Reproduce
Steps to reproduce the behavior:

1. Which Couper version? Run couper version or docker run --entrypoint=/couper avenga/couper version
0.8 2021-04-08 2a27495
2. Provide your configuration file *.hcl. Remove sensitive data.

server "couper" {

  endpoint "/" {
    request {
      backend = "ref"
    }
  }
}

definitions {
  backend "ref" {
    origin = "https://httpbin.org"
  }
}

3. Provide a curl call for reproduction
   curl -v http://localhost:8080/
4. See an error response or error log.
   ERRO[0018] connecting to default ":80" failed: dial tcp :80: connect: connection refused

Expected behavior
Connect and request https://httpbin.org/