Content Category Meta Desc and Meta Keywords don't work
Fixes #4012
Joomla! Tracker Item [#32864](http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=32864&start=0)

The meta description, meta keywords and author set in a Category are never used in the front-end when displaying a single category.

The components/com_content/views/category/view.html.php simply lacks any code whatsoever to handle these fields. This PR adds the missing code, using the same approach as the article view of com_content.