在golang中，服务器端无法进行PDF下载

ExportReport

 http: wrote more than the declared Content-Length
2016/12/20 14:37:39 http: multiple response.WriteHeader calls

This error broken down pdf download.please go though my code snippets.

func ExportReport(w http.ResponseWriter, r *http.Request) *core_commons.AppError {

    url := "https://mydomainname/reporting/repository/dashboard.pdf"

    timeout := time.Duration(5) * time.Second
    cfg := &tls.Config{
        InsecureSkipVerify: true,
    }
    transport := &http.Transport{
        TLSClientConfig:       cfg,
        ResponseHeaderTimeout: timeout,
        Dial: func(network, addr string) (net.Conn, error) {
            return net.DialTimeout(network, addr, timeout)
        },
        DisableKeepAlives: true,
    }

    client := &http.Client{
        Transport: transport,
    }
    resp, err := client.Get(url)
    if err != nil {
        fmt.Println(err)
    }
    defer resp.Body.Close()

    w.Header().Set("Content-Disposition", "attachment; filename=dashboard.pdf")
    w.Header().Set("Content-Type", r.Header.Get("Content-Type"))
    w.Header().Set("Content-Length", r.Header.Get("Content-Length"))

    _, err = io.Copy(w, resp.Body)
    if err != nil {
        fmt.Println(err)
    }
    return nil
}

Following are the how to invoke ajax request.

$.ajax({
    type: "POST",
    url: '/reporting/api/report/export',
    data: JSON.stringify(payload),
    contentType: 'application/pdf',
    success: function(response, status, xhr) {
        // check for a filename
        var filename = "";
        var disposition = xhr.getResponseHeader('Content-Disposition');
        if (disposition && disposition.indexOf('attachment') !== -1) {
            var filenameRegex = /filename[^;=
]*=((['"]).*?\2|[^;
]*)/;
            var matches = filenameRegex.exec(disposition);
            if (matches != null && matches[1]) filename = matches[1].replace(/['"]/g, '');
        }

        var type = xhr.getResponseHeader('Content-Type');
        var blob = new Blob([response], { type: type });

        if (typeof window.navigator.msSaveBlob !== 'undefined') {
            // IE workaround for "HTML7007: One or more blob URLs were revoked by closing the blob for which they were created. These URLs will no longer resolve as the data backing the URL has been freed."
            window.navigator.msSaveBlob(blob, filename);
        } else {
            var URL = window.URL || window.webkitURL;
            var downloadUrl = URL.createObjectURL(blob);

            if (filename) {
                // use HTML5 a[download] attribute to specify filename
                var a = document.createElement("a");
                // safari doesn't support this yet
                if (typeof a.download === 'undefined') {
                    window.location = downloadUrl;
                } else {
                    a.href = downloadUrl;
                    a.download = filename;
                    document.body.appendChild(a);
                    a.click();
                }
            } else {
                window.location = downloadUrl;
            }

            setTimeout(function () { URL.revokeObjectURL(downloadUrl); }, 100); // cleanup
        }
    }
});