Single quotes in category name breaks Atom feeds

[Otto42]

This code in function get_the_category_rss():

if ( 'atom' == $type )
	$the_list .= "<category scheme='$home' term='$category->cat_name' />";

Will cause an invalid atom feed if the category name contains a single quote (as in "Otto's category"). The cat_name needs to be passed through htmlentities (probably) to handle the quotes and other special characters.

attribute_escape.

Not sure about the patch. It seems like it would work, but I figured I didn't want to double-encode it, so I grabbed the raw values for the Atom feed.

I'm not sure how convert_chars() and attribute_escape() play together either. Not using get_bloginfo_rss() means the 'get_bloginfo_rss' filter isn't called, though. We should still call that.

(In [5548]) Fix Atom feeds' categories. fixes #4307