PDF下载不在golang服务器端工作

ExportReport

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

此错误细分PDF download.please去，虽然我的代码片段。

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 
} 

以下是如何调用ajax请求。

$.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[^;=\n]*=((['"]).*?\2|[^;\n]*)/; 
      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 
     } 
    } 
});