title: default image for paperclip is not displayed
body: <p>While looping through my objects in a view I am trying to display the first attached image of my listing like this:</p>
  
  <pre><code>&lt;%= image_tag listing.assets.first.attachment.url(:small) %&gt;
  </code></pre>
  
  <p>This works fine if all my listings have at least one image but if one listing doesn't have an image, the default image is not shown. Instead I get a undefined method `attachment' for nil:NilClass.</p>
  
  <p>Which is logical because the object is nil but I have this in my model</p>
  
  <pre><code>has_attached_file :attachment, :styles =&gt; { :medium =&gt; "300x300&gt;",
  :small =&gt; "200x200&gt;", :thumb =&gt; "100x100&gt;" }, :default_url =&gt;
  "no_image_:style.jpg"
  </code></pre>
  
  <p>So I assume if image_tag doesn't receive aI url it should display "no_image_small.jpg" which is in my <strong>app/assets/images</strong> folder.</p>
  
  <p>Is that how it should work? How can I display my no_image_small.jpg if the image doesn't exist and is nill?</p>