title: undefined method `asset_data_uri' - Rail 3.2
body: <p>I have a partial "person.css.erb":</p>
  
  <pre><code> #caption {
     background-image: url(&lt;%= asset_data_uri("caption.png") %&gt;); 
     text-align: center;
   }
  </code></pre>
  
  <p>When the partial is rendered it fails with:</p>
  
  <pre><code>undefined method `asset_data_uri'
  </code></pre>
  
  <p>The rails asset pipeline guide has an example of using this method: <a href="http://guides.rubyonrails.org/asset_pipeline.html" rel="nofollow">http://guides.rubyonrails.org/asset_pipeline.html</a></p>
  
  <p>Similar helpers work, eg. asset_path.   I'm using Rails 3.2.8.   Is the guide outdated?  Was the method renamed?  Do I need to do something special to get this helper included?</p>