Improve feed.xml.

* add `generator` property
* add image property
* use `xml_escape` instead of `![CDATA]`
* use `date_to_rfc822` instead of the date format string (thanks @troyswanson!)
* fix self reference url
* switch to `isPermaLink` for the `guid`

site/feed.xml

@@ -11,25 +11,38 @@ permalink: /feed.xml
   xmlns:sy="http://purl.org/rss/1.0/modules/syndication/"
   xmlns:slash="http://purl.org/rss/1.0/modules/slash/"
   >
-<channel>
+  <channel>
     <title xml:lang="en">Jekyll • Simple, blog-aware, static sites</title>
-    <atom:link type="application/atom+xml" href="http://jekyllrb.com/feed/" rel="self"/>
-    <link>http://jekyllrb.com</link>
-    <pubDate>{{ site.time | date: "%a, %d %b %Y %H:%M:%S %z" }}</pubDate>
-    <lastBuildDate>{{ site.time | date: "%a, %d %b %Y %H:%M:%S %z" }}</lastBuildDate>
+    <atom:link type="application/atom+xml" href="http://jekyllrb.com/feed.xml" rel="self"/>
+    <link>http://jekyllrb.com/</link>
+    <pubDate>{{ site.time | date_to_rfc822 }}</pubDate>
+    <lastBuildDate>{{ site.time | date_to_rfc822 }}</lastBuildDate>
     <language>en-US</language>
+    <generator>Jekyll v{{ jekyll.version }}</generator>
     <description>Transform your plain text into static websites and blogs.</description>
+    <image>
+      <description>Transform your plain text into static websites and blogs.</description>
+      <url>http://jekyllrb.com/img/logo-rss.png</url>
+      <title>Jekyll • Simple, blog-aware, static sites</title>
+      <link>http://jekyllrb.com/</link>
+      <width>144</width>
+      <height>73</height>
+    </image>
     {% for post in site.posts %}
     <item>
       <title>{{ post.title | xml_escape}}</title>
       <link>http://jekyllrb.com{{ post.url }}</link>
-        <pubDate>{{ post.date | date: "%a, %d %b %Y %H:%M:%S %z" }}</pubDate>
+      <pubDate>{{ post.date | date_to_rfc822 }}</pubDate>
       <dc:creator>{{ post.author }}</dc:creator>
-        {% for tag in post.tags %}<category>{{ tag | xml_escape }}</category>
-        {% endfor %}{% for cat in post.categories %}<category>{{ cat | xml_escape }}</category>
-        {% endfor %}<guid isPermaLink="false">{{ post.id }}</guid>
-        <description><![CDATA[ {{ post.content }} ]]></description>
+      {% for tag in post.tags %}
+      <category>{{ tag | xml_escape }}</category>
+      {% endfor %}
+      {% for cat in post.categories %}
+      <category>{{ cat | xml_escape }}</category>
+      {% endfor %}
+      <guid isPermaLink="true">http://jekyllrb.com{{ post.url }}</guid>
+      <description>{{ post.content | xml_escape }}</description>
     </item>
     {% endfor %}
-</channel>
+  </channel>
 </rss>