title: Model validation errors not appearing for nested model in simple_form
body: <p>So I have a simple form for entering "brand" and "model" through a single submit button, as below:</p>
  
  <pre><code>&lt;%= simple_form_for @brand, :html =&gt; { :class =&gt; 'form-horizontal' } do |m| %&gt;
    &lt;fieldset&gt;
      &lt;legend&gt;&lt;%= controller.action_name.capitalize %&gt; /Brand&lt;/legend&gt;
          &lt;%= m.input :name %&gt;
  
          &lt;%= m.simple_fields_for :models, Model.new do |p| %&gt;
        &lt;%= p.input :name %&gt;
      &lt;% end %&gt;
  
      &lt;div class="form-actions"&gt;
        &lt;%= m.submit nil, :class =&gt; 'btn btn-primary' %&gt;
        &lt;%= link_to 'Cancel', brands_path, :class =&gt; 'btn' %&gt;
      &lt;/div&gt;
    &lt;/fieldset&gt;
  &lt;% end %&gt;
  </code></pre>
  
  <p>I have name:string in my schema for both brand and model.. and validates_presence_of :name in both models.. The form DOES work for creating the brand and model simultaneously but my error "can't be blank" only shows up for the brand field. </p>
  
  <p>Thanks for any help with this issue.</p>