Fix 0008539: Special characters not parsed correctly in RSS output

PHP's DOMDocument::createTextNode automatically escapes special
characters within the node value. Therefore we don't need to double
escape these special characters (in particular, the ampersand) using
string_rss_links(...) when PHP already does the escaping for us.

Use of DOMDocument::createCDATASection doesn't escape special characters
and thus we still need to use string_rss_links(...) to do our own
escaping of the description field.