title: Override as_json or to_json model class name
body: <p>I'd like to modify the classname when calling to_json on an AR model.</p>
  
  <p>i.e.</p>
  
  <pre><code>Book.first.to_json
   #=&gt; "{\"book\":{\"created_at\":\"2010-03-23 
  
  Book.first.to_json(:root =&gt; 'libro')
   #=&gt; "{\"libro\":{\"created_at\":\"2010-03-23 
  </code></pre>
  
  <p>Is there an option to do this?</p>