title: JMS Serialize @VirtualProperty doesn't work
body: <p>I'm trying to add a method to serialize, but JMS Serialize does not show the field.</p>
  
  <pre><code>use JMS\Serializer\Annotation\Groups;
  use JMS\Serializer\Annotation\VirtualProperty;
  use JMS\Serializer\Annotation\SerializedName;
  
  class Ads
  {   
      /**
       * @VirtualProperty
       * @Type("string")
       * @SerializedName("Foo")
       * @Groups({"manage"})
       */
      public function foo(){
          $foo = 'foo';
          return $foo;
      }
      ...
  }
  </code></pre>
  
  <p>And then:</p>
  
  <pre><code>use JMS\Serializer\SerializationContext;
  use JMS\Serializer\SerializerBuilder;
  
  ...
  
  $context = new SerializationContext();
  $context-&gt;setGroups(array('manage'));
  
  $serializer = JMS\Serializer\SerializerBuilder::create()-&gt;build();
  $jsonContent = $serializer-&gt;serialize($ad, 'json', $context);
  </code></pre>
  
  <p>I have not seen any examples of how to use VirtualProperty.</p>
  
  <p>Is the syntax correct? What is wrong?</p>
  
  <p>Thank you.</p>