title: Symfony: Using a type='text' with form type 'checkbox'
body: <p>Is there a way to use the checkbox form type with a field that has been set to string?</p>
  
  <p>This field can hold many values, it can be used to input data as a textfield or as a checkbox.</p>
  
  <p>I have a event listener on the formtype to check if it should be a checkbox or text field.</p>
  
  <p>/Entity.php</p>
  
  <pre><code>/**
   * @var string 
   *
   * @ORM\Column(name="value", type="string", length=200, nullable=true)
   */
  private $value;
  </code></pre>
  
  <p>formtype.php</p>
  
  <pre><code>$form-&gt;add('value', 'checkbox', array(
  
  ))
  </code></pre>
  
  <p>Error</p>
  
  <pre><code>Unable to transform value for property path "value": Expected a Boolean.
  </code></pre>