req.json_body result is an empty object for specific types

[malud]

Describe the bug
The variable req.json_body does not return the expected outcome.

To Reproduce
Steps to reproduce the behavior:

1. Which Couper version? Run couper version or docker run --entrypoint=/couper avenga/couper version
master 2021-03-29 acfce46
2. Provide your configuration file *.hcl. Remove sensitive data.

server "api" {
  api {
    base_path = "/api"

    endpoint "/req" {
      response {
#        body = json_encode(req.json_body)
        body = json_encode([1,2,3])
      }
    }
  }
}

3. Provide a curl call for reproduction
   curl -s -H "Content-Type: application/json" -d "[1,2,3]" localhost:8080/api/req|jq

Expected behavior

• json_encode([1,2,3]): got expected

 [
  1,
  2,
  3
]

• json_encode(req.json_body) got empty object, expected array with 1, 2, 3

 {}

Additional context
This happens with boolean, number, string or array values.

[malud]

At the moment we assume an object instead of an array as root which is the cause for the empty result.