title: Rail + Compass sprite generation gives wrong image path
body: <p>In my Rails 3.2.1 application I am using compass(0.12.alpha.4) to create sprites:</p>
  
  <pre><code>@import compass
  
  @import "../images/category-icons/type/*.png"
  @include all-type-sprites
  //+all-type-sprites
  </code></pre>
  
  <p>The sprite image was genereated correctly, sitting in <em>app/assets/images/category-icons/type-sa69c4767de.png</em></p>
  
  <p>However the stylesheet generated is wrong. It points to <code>/assets/../images/category-icons/type-sa69c4767de.png</code>. It merely appends <code>assets</code> to the path. It works outside of Rails, so I think it has something to do with asset pipeline. What should I configure for this to work properly?</p>