title: Sunspot: can't use "order_by" on text fields?
body: <p>If I try to use <code>order_by</code> on a text field I get: "no field configured with name title".</p>
  
  <p>Works fine on fields of type "string". So I end up using a workaround like:</p>
  
  <pre><code>searchable do 
    text :title
  
    string :title_sortable do 
      title
    end
  end
  </code></pre>
  
  <p>Is there any reason why we cannot <code>order_by</code> on text fields?</p>