Redirect bare category URLs for permalink structures starting with %category%. Fixes #19876.

When using a structure like /%category%/%postname%/ it might be thought that /uncategorized/
(missing the "/category/" base) results in the category archive. Previously this worked due to
$walk_dirs = true for the post permalink structure, but canonical didn't redirect it. Now
there is no rewrite rule to match => 404. The fix is to use a canonical redirect.


git-svn-id: https://core.svn.wordpress.org/trunk@19778 1a063a9b-81f0-0310-95a4-ce76da25c4cd