title: Upgrade to Rails 3.1.0 from rc6, asset precompile fails
body: <p>After I upgrade to Rails 3.1.0 from Rails 3.1.0.rc6, I got some error messages when running <code>assets:precompile</code> task like this:</p>
  
  <pre><code>$ RAILS_ENV=production RAILS_GROUPS=assets rake assets:clean tmp:clear assets:precompile
  rake aborted!
  ie-spacer.gif isn't precompiled
    (in app/assets/stylesheets/jquery.gritter.css.scss)
  
  Tasks: TOP =&gt; assets:precompile
  (See full trace by running task with --trace)
  </code></pre>
  
  <p>I tried add <code>config.assets.precompile &lt;&lt; 'ie-spacer.gif'</code> to <code>config/application.rb</code>, but it still fails.</p>
  
  <p>The line in <code>jquery.gritter.css.scss</code> is:</p>
  
  <pre><code>background: asset_url('ie-spacer.gif', image);
  </code></pre>
  
  <p>and I tried:</p>
  
  <pre><code>background: asset_path('ie-spacer.gif', image);
  background: image_url('ie-spacer.gif');
  background: image_path('ie-spacer.gif');
  </code></pre>
  
  <p>all failed with <code>ie-spacer.gif isn't precompiled</code>.</p>