title: Zend validate interprets "disabled" input fields as empty
body: <p>I again ran into a problem I just can´t seem to comprehend. 
  I have a form element like so:</p>
  
  <pre><code>$this-&gt;add(array(
          'type'    =&gt; 'datetime',
          'name'    =&gt; 'modifiedTime',
          'options' =&gt; array(
              'label' =&gt; 'Modified Time',
              ),
           'attributes' =&gt; array(
              'disabled' =&gt; 'disable',
              ),
  
          ));
  </code></pre>
  
  <p>This one does get filled correctly trough my entity(I am using doctrine) like so:</p>
  
  <pre><code>/**
   * @ORM\Column(type="datetime", nullable=true, name="modified_time") 
   * 
   * @Form\Exclude()
   */
  protected $modifiedTime;
  
  public function getModifiedTime(){
      return $this-&gt;modifiedTime;
  }
  
  public function populate($data)
  {
  $this-&gt;modifiedTime = date_create($data['modifiedTime']);
  }
  </code></pre>
  
  <p>This works completly fine as long as the "disabled" attribute is not set. But as soon as it is I get a validation error claiming "Value is required and can't be empty" eventhou the value is set in the input.</p>
  
  <p>any Ideas?</p>