backend response status not a number

[johakoch]

Describe the bug
The status from an upstream response cannot be set as status of the client response.

To Reproduce
Steps to reproduce the behavior:

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

0 2021-03-31 dev

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

server "test" {

  api {
    endpoint "/headers" {
      request "second" {
        url = "https://httpbin.org/status/404"
      }
      response {
        status = beresps.second.status
      }
    }
  }
}

3. Provide a curl call for reproduction

$ curl -i localhost:8080/headers

4. See an error response or error log.

INFO[0004]                                               backend=default build=dev realtime=660.018 request="map[addr:httpbin.org headers:map[] host:httpbin.org method:GET name:second path: proto:HTTP/1.1 scheme:https]" response="map[headers:map[content-type:text/html; charset=utf-8] proto:HTTP/1.1 tls:true]" status=404 timings="map[connect:112.443 dns:10.312 tls:422.118 ttfb:114.088]" type=couper_backend uid=c1i34j5916bipub2hp1g url="https://httpbin.org:443/status/404" version=0
2021/03/31 10:19:09 http: panic serving 127.0.0.1:65225: not a number
goroutine 34 [running]:
net/http.(*conn).serve.func1(0xc000380000)
        /usr/local/go/src/net/http/server.go:1801 +0x147
panic(0xa4f200, 0xbdfc90)
        /usr/local/go/src/runtime/panic.go:975 +0x47a
github.com/zclconf/go-cty/cty.Value.AsBigFloat(0xbfbbe0, 0xc000026189, 0xa4f200, 0xc000478dd0, 0xc000026101)
        /c/Users/johannes.koch.SEVENVAL/src/couper/vendor/github.com/zclconf/go-cty/cty/value_ops.go:1271 +0x1f0
github.com/avenga/couper/internal/seetie.ValueToInt(0xbfbbe0, 0xc000026189, 0xa4f200, 0xc000478dd0, 0xa4f200)
        /c/Users/johannes.koch.SEVENVAL/src/couper/internal/seetie/convert.go:217 +0x85
github.com/avenga/couper/handler.(*Endpoint).newResponse(0xc000324050, 0xc000396200, 0xc000092230, 0x2, 0xc000092230, 0xc000396200)
        /c/Users/johannes.koch.SEVENVAL/src/couper/handler/endpoint.go:182 +0x771
github.com/avenga/couper/handler.(*Endpoint).ServeHTTP(0xc000324050, 0xbf8c20, 0xc0003a4080, 0xc000396200)
        /c/Users/johannes.koch.SEVENVAL/src/couper/handler/endpoint.go:107 +0x8a6
github.com/avenga/couper/logging.(*AccessLog).ServeHTTP(0xc000323440, 0xbf8c60, 0xc0003a6000, 0xc000396200, 0xbed6c0, 0xc000324050, 0xc01129ff1e0da700, 0xb5898771, 0xf71800)
        /c/Users/johannes.koch.SEVENVAL/src/couper/logging/access_log.go:42 +0x22d
github.com/avenga/couper/server.(*HTTPServer).ServeHTTP(0xc000148980, 0xbf91e0, 0xc0003a0000, 0xc000396000)
        /c/Users/johannes.koch.SEVENVAL/src/couper/server/http.go:210 +0x52a
net/http.serverHandler.ServeHTTP(0xc00012a9a0, 0xbf91e0, 0xc0003a0000, 0xc000396000)
        /usr/local/go/src/net/http/server.go:2843 +0xa3
net/http.(*conn).serve(0xc000380000, 0xbfb420, 0xc00038e000)
        /usr/local/go/src/net/http/server.go:1925 +0x8ad
created by net/http.(*Server).Serve
        /usr/local/go/src/net/http/server.go:2969 +0x36c

Expected behavior
The status from a backend response should be a number, so that it can be used as status for the client response.