title: Symfony 3 restrict controller action to to consume specific content type
body: <p>can be controller action in SF3 annotated to consume specific 'Content-Type'?</p>
  
  <p>I'am trying to abandon '/api/post/{id}/xml' route hack.</p>
  
  <pre><code>namespace AppBundle\Controller;
  use Sensio\Bundle\FrameworkExtraBundle\Configuration\Method;
  class BlogApiController extends Controller
  {
  /**
   * @Route("/api/posts/{id}")
   * @Method({"GET","HEAD"})
   * some-magic-annotation-here
   */
  public function showJson($id)
  {
      // response in json
  }
  
  /**
   * @Route("/api/posts/{id}")
   * @Method({"GET","HEAD"})
   * some-magic-annotation-here
   */
  public function showXml($id)
  {
      // response in XML
  }
  }
  </code></pre>