title: image tag does not include asset digest on email
body: <p>I have a Rails 4 app that sends out some HTML emails. The emails contains an image in the footer but I have an hard time to properly render it on production.
  I have looked on stackoverflow several answers about this topic and everyone says </p>
  
  <p>1 - to add <code>config.action_mailer.asset_host = "http://example.com"</code></p>
  
  <p>2 - to use the helper method <code>image_tag</code> to render the images</p>
  
  <p>So I have done that </p>
  
  <pre><code>#config/environments/production.rb
  ...
  config.action_mailer.asset_host = "http://example.com"
  
  #app/views/app_mailer/layout/dark.html.erb
  ...
  &lt;%= image_tag "dark/footer.png", style: "border: 0;", alt: "footer" %&gt;
  </code></pre>
  
  <p>However the generated src tag doesn't take in consideration the digest postfixed to the image name once I precompile the assets.</p>
  
  <p>In my email I get <code>src="http://example.com/assets/dark/footer.png"</code> </p>
  
  <p>and in my app I have <code>public/assets/dark/footer-e2f622dcfd3016d12ec655bcfe81ec3d.png</code></p>
  
  <p>so the image is not properly rendered.</p>
  
  <p>Am I missing anything here?</p>
  
  <p>Thanks.</p>