docs: private HttpURLConnection openHttp() throws IOException {
  		final URLConnection conn = openConnection();
  		if (false == conn instanceof HttpURLConnection) {
  			// 防止其它协议造成的转换异常
  			throw new HttpException("'{}' is not a http connection, make sure URL is format for http.", conn.getClass().getName());
  		}
  
  		return (HttpURLConnection) conn;
  	}