title: Adding custom attributes on to_json
body: <p>While using <a href="https://github.com/vigetlabs/serialize_with_options" rel="nofollow">serialize_with_options</a> I came to realise that it doesn't work as I expected when it comes to arrays.</p>
  
  <p>So given an array of @posts (as seen in the README) calling @posts.to_json will neither include the user or show only the title.</p>
  
  <p>Not sure if that's the expected behaviour or I'm missing something since I can't find anything related.</p>
  
  <p>Using Rails 3.0.4</p>
  
  <p>PS. Are there any alternatives when it comes to include 2 custom attributes on the JSON format of a model?</p>