SetRoundTripper not effective

[darren]

package main

import (
	"log"
	"net"
	"net/http"
	"net/url"

	"github.com/google/martian/v3"
)

func main() {
	p := martian.NewProxy()
	p.SetRoundTripper(
		&http.Transport{
			Proxy: func(r *http.Request) (*url.URL, error) {
				log.Printf("%v", r.URL)
				return nil, nil
			},
		},
	)
	lis, _ := net.Listen("tcp", ":8080")
	p.Serve(lis)
}

curl -x 127.0.0.1:8080 http://www.google.com/

should print http://www.google.com/, but none is print. is this working as expected?