Update libraries/joomla/document/feed/renderer/rss.php

Fix for:
RSS feed gets corrupted when ampersand (&) or other escapable characters exist in Site Meta Description

Bug filed here:
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_id=8103&tracker_item_id=29731

Bug also submitted (and now merged) to Joomla platform:
https://github.com/joomla/joomla-platform/pull/1716
https://github.com/joomla/joomla-platform/pull/1716.patch
https://github.com/joomla/joomla-platform/pull/1716.diff