Update the rss posts updated time

To hint crawlers that the posts are updated

atom.xml

@@ -19,7 +19,7 @@ layout: null
  <entry>
    <title>{{ post.title }}</title>
    <link href="{{ site.url }}{{ post.url }}?ref=rss"/>
-   <updated>{{ post.date | date_to_xmlschema }}</updated>
+   <updated>{{ site.time | date_to_xmlschema }}</updated>
    <id>{{ site.url }}{{ post.id }}</id>
    <content type="html">
     {% if post.content contains "<!-- more -->" %}