title: Image assets not found after upgrading to Rails 3.1
body: <p>I've upgraded an app to Rails 3.1, and everything is setup and working correctly except that an image_tag isn't finding the image.  I've moved all my images from public/images to app/assets/images.  The browser requests an image at <a href="http://localhost:3000/assets/foobar.png">http://localhost:3000/assets/foobar.png</a> but it just shows up as a broken image in Chrome.  Opening it in a separate tab says "No route matches [GET] "/assets/foobar.png".</p>
  
  <p>I feel like I'm just missing some setting somewhere, but I haven't been able to find out what it is yet.  Any suggestions?</p>