taxonomy.module should run check_plain on rss output

In the case of a category name with an amphersand or other reserved xml character the rss output of taxonomy module can break the feed. This one line patch runs check_plain on the category value in the taxonomy_rss_item() function which is the way that node_feed works and solves the problem. This patch is against cvs, can provide one for 4.6 if needed.

Thanks!