title: custom sprockets preprocessor & asset precompiling
body: <p>I am having an issue with using custom sprockets preprocessors when precompiling assets in rails 3.2.3</p>
  
  <p>I am using the <a href="https://github.com/carlhoerberg/sprockets-urlrewriter" rel="nofollow">sprockets-urlrewriter</a> along with twitter bootstrap.</p>
  
  <p>In development, the preprocessor declared in an initializer is working just fine.  <code>Rails.application.assets.register_preprocessor 'text/css', Sprockets::UrlRewriter</code></p>
  
  <p>It successfully converts the relative URLs in my CSS <code>url(\"../img/glyphicons-halflings.png\")</code> to <code>url(bootstrap/img/glyphicons-halflings.png)</code></p>
  
  <p>wonderful.</p>
  
  <p>But when I precompile my assets: <code>rake assets:precompile:all RAILS_ENV=production RAILS_GROUPS=assets</code>, the compiled css has the original relative path.  How can i troubleshoot precompiling my assets with the preprocessor?</p>