Notice: Undefined variable: query_string in cache_actions_rules_cache_get_id()

Problem/Motivation

Adding text to a undeclared variable leads to annoying PHP notices.

Proposed resolution

simple patch to init the variable before actually adding content to it.

 foreach ($query as $key => $value) {

    // just a little check
    if(!isset($query_string)){
      $query_string = '';
    }
    // go on, chain away now!
    $query_string .= $key . '=' . $value . '&';
  }