title: Shortcode in iframe url
body: <p>im trying to use shortcode for tracking in iframe and links urls but iframe is ignoring the shortcode. Can some1 help me with that? Thanks!<br>
  I have shortcode function like this:</p>
  
  <pre><code>function myShortcode(){
    return 'test';
  }
  add_shortcode( 'tracking', 'myShortcode' );
  </code></pre>
  
  <p>if i put link to content like this:</p>
  
  <pre><code>&lt;a href="http://example.com/?tracking=[tracking]"&gt;Tracking link&lt;/a&gt;
  </code></pre>
  
  <p>it returns link with tracking info like: <a href="http://example.com/?tracking=test" rel="nofollow">http://example.com/?tracking=test</a> (correct)</p>
  
  <p>but when i use it in iframe like this</p>
  
  <pre><code>&lt;iframe src="http://example.com/iframe/?tracking=[tracking]"&gt;&lt;/iframe&gt;
  </code></pre>
  
  <p>it return link inside like this: <a href="http://example.com/iframe/?tracking=[tracking]" rel="nofollow">http://example.com/iframe/?tracking=[tracking]</a> (incorrect - missing 'test' value)</p>