Outdated GRDDL profile URL

Problem/Motivation

The default GRDDL profile URL set in Drupal core is outdated. It is currently set to http://www.w3.org/1999/xhtml/vocab, but this URL is a permanent redirect to http://www.w3.org/1999/xhtml/vocab/ (note the trailing slash).

Although this link is typically ignored by browsers and search engines, some other tools still follow it and detect an error, for example the W3C Link Checker.

For example, link-validating a fresh Drupal 7 website with the default Bartik theme, such as http://demo.opensourcecms.com/drupal/, reports something like:

Line: 14 http://www.w3.org/1999/xhtml/vocab redirected to http://www.w3.org/1999/xhtml/vocab/
Status: 301 -> 200 OK

The link is missing a trailing slash, and caused a redirect. Adding the trailing slash would speed up browsing.

Proposed resolution

The GRDDL profile needs to be updated to the current URL in theme.inc. The following patch does exactly this.