Fix double encoding in rss syntax FS#2731

The new SimplePie version not only applies htmlspecialchars() but creates a DOM document which applies all sorts of HTML entities. This means that for example "ä" will become '"&amp;auml;'". htmlspecialchars_decode() does not decode these entities which leads to double encoding.

As far as I understood the documentation html_entity_decode() does everything htmlspecialchars_decode() does and in addition it decodes HTML entities, that's why I propose in this pull request to replace htmlspecialchars_decode() by html_entity_decode().