contrib/go-chi/chi: Handle 0 status code correctly

[adw1n]

When the user doesn't call w.WriteHeader(...) or w.Write(...) in the request handler or any of the middlewares the status isn't set and WrapResponseWriter.Status() returns 0. This is well documented behavior: https://github.com/go-chi/chi/blob/cf97bef8b1405c9177bd88eca166ed4f16b9fc80/middleware/wrap_writer.go#L45

In such a case net/http library takes the matters into its own hands and writes 200 status code: https://github.com/golang/go/blob/7e9369a517d9ebf867748719948d8cbccec3bc57/src/net/http/server.go#L1618

Try this code:

package main

import (
	"net/http"

	"github.com/go-chi/chi"
	"github.com/go-chi/chi/middleware"
)

func main() {
	r := chi.NewRouter()
	r.Use(middleware.Logger)
	r.Get("/", func(w http.ResponseWriter, r *http.Request) {
	})
	http.ListenAndServe(":3000", r)
}

with:

curl -i http://localhost:3000

Result:

HTTP/1.1 200 OK
Date: Mon, 21 Sep 2020 12:00:00 GMT
Content-Length: 0

Observe the 000 status code logged by chi's logging middleware that similarly relies on ww.Status() https://github.com/go-chi/chi/blob/cf97bef8b1405c9177bd88eca166ed4f16b9fc80/middleware/logger.go#L43:

2020/09/21 12:00:00 "GET http://localhost:3000/ HTTP/1.1" from [::1]:50000 - 000 0B in 10.001µs

Without changes proposed in this PR traces have empty status code in similar cases.

[knusbaum]

@adw1n Fantastic. Thank you for catching this edge case.

Do you have time to add a quick unit test for this? If not I can add one and we can get this in.

[adw1n]

@knusbaum done

[knusbaum]

LGTM!