message: fixed Connection up_msg_handler url
diff: @@ -53,7 +53,7 @@ impl<UMsg: Serialize, DMsg: DeserializeOwned> Connection<UMsg, DMsg> {
  // ---- Request ----
  let request =
  - Request::new_with_str_and_init("_api/up_msg_handler", &request_init).unwrap_throw();
  + Request::new_with_str_and_init("/_api/up_msg_handler", &request_init).unwrap_throw();
  // ---- Headers ----
  let cor_id = CorId::new();
  