Update Router.php

Router::reverse() is really useful for generating string URLs, but sometimes I want to submit $params that are a modified instance of CakeRequest. For example, I may use Router::parse() to break down a URL, and then I may want to manually add/remove some ['named'] or ['pass'] info and then turn it back into a string.  If I submit that modified array, it's not an instance of CakeRequest, so it looks for $params['url'], which also doesn't exist.  This change just makes it skip the Notice error that is throw, without changing the functionality.