如何在HTTP请求中发送多部分表单数据（用于Watson NLC培训）？

我正在编写将使用Watson Natural Language Classifier（“ NLC”）的应用程序。当我使用以下请求消息正文针对v1 / classifiers URI路径发送HTTP POST请求时，服务器将以状态码415（不支持的媒体类型）进行响应：

--04fef47728eb08148fe9c7b18dd42b75abd75ebf752fd3412a85aa3af075
Content-Disposition: form-data; name="training_data"; filename="data.csv"
Content-Type: text/csv

How hot is it today?;temperature
Is it hot outside?;temperature
Will it be uncomfortably hot?;temperature
Will it be sweltering?;temperature
How cold is it today?;temperature
Is it cold outside?;temperature
Will it be uncomfortably cold?;temperature
Will it be frigid?;temperature
What is the expected high for today?;temperature
What is the expected temperature?;temperature
Will high temperatures be dangerous?;temperature
Is it dangerously cold?;temperature
When will the heat subside?;temperature
Is it hot?;temperature
Is it cold?;temperature
How cold is it now?;temperature
Will we have a cold day today?;temperature
When will the cold subside?;temperature
What highs are we expecting?;temperature
What lows are we expecting?;temperature
Is it warm?;temperature
Is it chilly?;temperature
What's the current temp in Celsius?;temperature
What is the temperature in Fahrenheit?;temperature
--04fef47728eb08148fe9c7b18dd42b75abd75ebf752fd3412a85aa3af075
Content-Disposition: form-data; name="training_metadata"; filename="metadata.json"
Content-Type: application/json

{"language": "en"}

415状态代码表明内容类型有问题，但似乎所有MIME类型都是正确的。

我的代码（用Go编写）：

func (w WatsonClassifier) createFormFile(writer *multipart.Writer, fieldname string, filename string, contentType string) (io.Writer, error)     {
    h := make(textproto.MIMEHeader)
    h.Set("Content-Disposition",
        fmt.Sprintf(`form-data; name="%s"; filename="%s"`,
            fieldname, filename))
    h.Set("Content-Type", contentType)
    return writer.CreatePart(h)
}

func (w WatsonClassifier) request(method string, apiUrl string, body io.Reader) (string, error) {
    url := w.url + "/" + apiUrl
    req, err := http.NewRequest(method, url, body)
    if err != nil {
        return "", err
    }
    req.SetBasicAuth(w.username, w.password)
    client := http.Client{}
    resp, err := client.Do(req)
    if resp.StatusCode != 200 {
        answer, _ := ioutil.ReadAll(resp.Body)
        fmt.Println(string(answer))
        return "", errors.New("Watson returned wrong status code : " + resp.Status)
    }
    answer, err := ioutil.ReadAll(resp.Body)
    if err != nil {
        return "", err
    }
    return string(answer), nil
}

func (w WatsonClassifier) Train(data []ClassifierCategory) (Classifier, error) {
    table := w.buildTable(data)
    str := w.buildCsv(data)
    buf := new(bytes.Buffer)
    writer := multipart.NewWriter(buf)
    data_part, err := w.createFormFile(writer, "training_data", "data.csv", "text/csv")
    if err != nil {
        return WatsonClassifier{}, err
    }
    data_part.Write([]byte(str))
    metadata_part, err := w.createFormFile(writer, "training_metadata", "metadata.json", "application/json")
    if err != nil {
        return WatsonClassifier{}, err
    }
    metadata_json := "{\"language\": \"" + w.Language + "\"}"
    metadata_part.Write([]byte(metadata_json))
    fmt.Println(buf.String())
    answer, err := w.request("POST", "v1/classifiers", buf)
    if err != nil {
        return WatsonClassifier{}, err
    }
    fmt.Println(answer)
    return WatsonClassifier{}, nil
}