[BUG] http: ContentLength=xx with Body length 0

[ioito]

Describe the bug
A clear and concise description of what the bug is.

after #31 merged

client.Do(req) https request will return http: ContentLength=xx with Body length 0 error after http2curl.GetCurlCommand(req) called

To Reproduce

str := `{"hello":"world"}`
req, _ := http.NewRequest("POST", `https://www.baidu.com`, strings.NewReader(str))
req.Header.Set("Content-Length", strconv.FormatInt(int64(len(str)), 10))
client := &http.Client{
    Transport: &http.Transport{
        TLSClientConfig: &tls.Config{InsecureSkipVerify: true},
    },
}
http2curl.GetCurlCommand(req)
_, err := client.Do(req)
if err != nil {
    fmt.Println("err: ", err)
}

/cc @Dzalevski

[djale1k]

@ioito @moul Yes. GetCurlCmd after reading from the body will be empty it :) -> it was made for a performance boost.
maybe you can make a duplicate buff for req.