Fixed, the atom feed does not convert relative URLs to absolute URLs.

The atom feed, as opposed to the rss feed, does not convert relative URLs to absolute URLs and causes broken links. This PR fixes the issue.

JC tracker: http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33842
can we write in somewhere in this docblock that it's used for the feed renderers for future reference please