title: Forcing required format in Twig generated URL
body: <p>I'm forcing <code>XML</code> format for a certain action.</p>
  
  <pre class="lang-php prettyprint-override"><code>/**
   * @Route(
   *      "/index.{_format}",
   *      name         = "index",
   *      requirements = {"_format" = "xml"},
   *      defaults     = {"_format" = "xml"}
   * )
   *
   * @Template
   */
  public function index()
  {
      // ...
  }
  </code></pre>
  
  <p>Then on some Twig template I'm generating an absolute URL for this action.</p>
  
  <pre><code>{{ url('index') }}
  </code></pre>
  
  <p>The result is:</p>
  
  <pre><code>http://www.domain.com/index
  </code></pre>
  
  <p>But I expected:</p>
  
  <pre><code>http://www.domain.com/index.xml
  </code></pre>
  
  <p>How can I achieve the expected behaviour?</p>