add a {X-,}Idempotency-Key header for http POST

fix https://github.com/chrislusf/seaweedfs/issues/1592

according to https://github.com/golang/go/issues/19943

weed/operation/upload_content.go

@@ -190,6 +190,7 @@ func upload_content(uploadUrl string, fillBufferFunction func(w io.Writer) error
 	body_writer := multipart.NewWriter(buf)
 	h := make(textproto.MIMEHeader)
 	h.Set("Content-Disposition", fmt.Sprintf(`form-data; name="file"; filename="%s"`, fileNameEscaper.Replace(filename)))
+	h.Set("Idempotency-Key", uploadUrl)
 	if mtype == "" {
 		mtype = mime.TypeByExtension(strings.ToLower(filepath.Ext(filename)))
 	}