Update UrlHelper.php

Make URL recognition more conformant with RFC3986 and RFC1738 network-path references (URI beginning with a doubleslash).

When used with WP-Piwik in a Wordpress installation that uses such network-path references like '//domain' as base URI, Piwik does't recognize such a URI as valid, consequently putting a "http://" in front of it, resulting in a 'http:////domain' pattern which confuses the WP-Piwik plugin, resulting in multiple rewrites and creations of new sites.