title: asset_path in Rails 4 is producing unhashed asset paths
body: <p>I've got a Rails 4.0.3 application, and I've got a pretty simple image tag:</p>
  
  <pre><code>&lt;%= image_tag('logo_big.png') %&gt;
  </code></pre>
  
  <p>When in development, this works as expected and everything is happy. However, in production, the tag still links to the unhashed version of the image:</p>
  
  <pre><code>&lt;img alt="Logo big" src="/z/logo_big.png"&gt;
  </code></pre>
  
  <p>It should be appending the hash to the filename, in this case resulting in "logo_big-21d3b1db0a8536f5717646dfea417791.png". Using asset_path directly also does not work. Javascript and CSS file links all work correctly and append the hash as normal.</p>
  
  <p>So why is it not including the hash?</p>
  
  <p>Additional info / confounding factors:</p>
  
  <ul>
  <li>I've got an alternate assets prefix ("asset" is a resource in my application), which is "z"</li>
  <li>I'm using Sass 3.2.19 (sass-rails 4.0.2)</li>
  <li>I'm using Compass 0.12.5 (compass-rails 1.1.6)</li>
  <li>Hashing works for images in every template, <em>except</em> for layout templates.</li>
  </ul>