title: FOSUserBundle deserialization to user object throws "you must define a type"
body: <p>Basically I want to deserialize json to user object. Like this</p>
  
  <pre><code>$serializer = $this-&gt;get('jms_serializer');
  
  $data = $serializer-&gt;deserialize($this-&gt;get('request')-&gt;getContent(),
                                   'My\FooBundle\Entity\User', 'json');
  </code></pre>
  
  <p>The request data passed to deserializer:</p>
  
  <pre><code>{"id":2,"username":"...@gmail.com","username_canonical":"...@gmail.com", ... }
  </code></pre>
  
  <p>Thrown error:</p>
  
  <blockquote>
    <p>request.CRITICAL: Uncaught PHP Exception
    JMS\Serializer\Exception\RuntimeException: "You must define a type for
    FOS\UserBundle\Model\User::$username." at
    ...\vendor\jms\serializer\src\JMS\Serializer\GenericDeserializationVisitor.php
    line 170 [] []</p>
  </blockquote>
  
  <p>I use sf 2.3. Is there anything that has to be setup for deserialization of fos user class? Any idea why I get this message and how to fix it?</p>