title: setting target attribute of form in Symfony2 FormBuilder
body: <p>I need to set my form's <code>target</code> attribute in <code>Symfony2</code> using <code>FormBuilder</code>.<br>
  I tried this:  </p>
  
  <pre><code>$fb = $this-&gt;createFormBuilder();
  $fb-&gt;setAttribute('target', '_top');
  </code></pre>
  
  <p>but it doesnt work!<br>
  Why?<br>
  How can I do this?</p>