Code:

package main

import (
    "bytes"
    "fmt"
    "net/http"
    "net/http/httptest"
    "strings"
)

func getBody(s string) (body []string) {
    pastHeaders := false
    for _, part := range strings.Split(s, "\r\n") {
        if pastHeaders {
            body = append(body, part)
        } else if part == "" {
            pastHeaders = true
        }
    }
    return
}

func main() {
    // Setup test server.
    ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        w.Write([]byte("Hello, Client!"))
    }))
    defer ts.Close()

    // Setup instrumented client.
    type record struct {
        o   Origin
        d   string
    }
    var records []*record
    client := http.Client{
        Transport: &Transport{
            Callback: func(o Origin, data []byte, err error) {
                records = append(records, &record{o, string(data)})
            },
        },
    }

    _, err := client.Post(ts.URL, "test", bytes.NewBufferString("Hail, Server!"))
    if err != nil {
        return
    }

    // There should be two records: request and response.
    for idx, r := range records {
        fmt.Printf("%d) %s\n", idx, r.o)
        for _, line := range getBody(r.d) {
            fmt.Println(line)
        }
    }

}