title: Pre-populate form value of unmapped field
body: <p>I have a form that is bound to an entity, but it also has an extra unmapped field:
  (from the FormType class)</p>
  
  <pre><code>$builder
      -&gt;add('name')
      -&gt;add('qoh')
      -&gt;add('serialNumber', 'text', array('mapped' =&gt; false, 'required' =&gt; false))
  </code></pre>
  
  <p>I want to pre-populate the serialNumber field from the controller with information taken from the request URL. The closest method I have found would be:</p>
  
  <p><code>$form-&gt;setData(mixed $modelData)</code></p>
  
  <p>but the API does not specify what form '$modelData' takes and nothing I've tried has had any effect.</p>