Escape post.title in atom.xml.

If post.title were to contain one of '&', '<', or '>', the XML would
become invalid. xml_escape is a Jekyll extension of Liquid to safely
escape values.

Using straight Liquid, this could also be done with:

<title type="html">
    {{ post.title | escape
                  | replace:'&','&amp;'
                  | replace:'<','&lt;'
                  | replace:'>','&gt;' }}
</title>

(Note, type=html and then the additional XML escaping.)

atom.xml

@@ -16,7 +16,7 @@ layout: nil
 
  {% for post in site.posts %}
  <entry>
-   <title>{{ post.title }}</title>
+   <title>{{ post.title | xml_escape }}</title>
    <link href="http://blog.rubygems.org{{ post.url }}"/>
    <updated>{{ post.date | date_to_xmlschema }}</updated>
    <id>http://blog.rubygems.org{{ post.id }}</id>