Set updated date for items in feeds

RSS doesn't support updated date for items, so we use the ATOM extension.
Updated dates also bump the global update

tpl/feed.atom.html

@@ -27,7 +27,8 @@
       {/if}
       <id>{$value.guid}</id>
       {if="$show_dates"}
-        <updated>{$value.iso_date}</updated>
+        <published>{$value.pub_iso_date}</published>
+        <updated>{$value.up_iso_date}</updated>
       {/if}
       <content type="html" xml:lang="{$language}">
         <![CDATA[{$value.description}]]>

tpl/feed.rss.html

@@ -22,7 +22,8 @@
           <link>{$value.url}</link>
         {/if}
         {if="$show_dates"}
-          <pubDate>{$value.iso_date}</pubDate>
+          <pubDate>{$value.pub_iso_date}</pubDate>
+          <atom:modified>{$value.up_iso_date}</atom:modified>
         {/if}
         <description><![CDATA[{$value.description}]]></description>
         {loop="$value.taglist"}