title: twig renders number field as text
body: <p>In my form class I added number field:</p>
  
  <pre><code>$builder-&gt;add('total_amount', 'number', array('required' =&gt; false));
  </code></pre>
  
  <p>In my form template:</p>
  
  <pre><code>{{ form_widget(form.total_amount, {'attr': {'class': 'field number_field'}}) }}
  </code></pre>
  
  <p>However, my page source shows:</p>
  
  <pre><code>&lt;input type="text" id="ResearchDataForm_total_amount" 
      name="ResearchDataForm[total_amount]" class="field number_field" value="0" /&gt;
  </code></pre>
  
  <p>I expected type="number", but it is rendered as type="text". Why so?</p>