title: Open graph tags not working in Blogger
body: <p>This are the open graph tags that i used inside the <code>&lt;/head&gt;</code> tag of my Blogger blog :</p>
  
  <pre><code>&lt;meta expr:content='data:post.dateHeader' property='article:published_time'/&gt;
  &lt;meta expr:content='data:post.lastUpdatedISO8601' property='article:modified_time'/&gt;
  &lt;meta expr:content='data:label.name' property='article:tag'/&gt;
  </code></pre>
  
  <p>but from the page source of one of my blog posts i got this :</p>
  
  <pre><code>&lt;meta content='' property='article:published_time'/&gt;
  &lt;meta content='' property='article:modified_time'/&gt;
  &lt;meta content='' property='article:tag'/&gt;
  </code></pre>