title: form doesn't work after use render
body: <p>I trying to use render method inside my activeAdmin form method, but after insert render
  in code, it stopped to work.</p>
  
  <pre><code>form do |f|
      f.inputs I18n.t('sale_header') do
        f.input :client
        f.input :room
      end
  
      f.inputs I18n.t('sale_items')  do
        render :partial =&gt; "form_sale"
      end
  
      f.inputs I18n.t('totalization') do
        f.input :sub_total, :input_html =&gt; { :disabled =&gt; :true }
        f.input :discount
        f.input :total_value, :input_html =&gt; { :disabled =&gt; :true }
      end
  
      f.buttons
  end
  </code></pre>
  
  <p>After insert the render method, only form_sale content is showed on screen.</p>
  
  <p>Any help?
  Thank You!</p>