Fix RSS feed published date

Author: mtrythall

src/feed.html

@@ -23,7 +23,7 @@
     <entry>
       <title>{{ post.data.title }}</title>
       <link href="{{ site.domain }}{{ post.url }}" />
-      <updated>{{ post.date | dateToRfc3339 }}</updated>
+      <updated>{{ post.data.published_datetime | dateToRfc3339 }}</updated>
       <id>{{ post_url }}</id>
       <content xml:lang="en" type="html"
         >{{ post.templateContent| escape }}</content