title: Symfony+JMS Serializer deserialize into existing object
body: <p>I've been wondering if it's possible to use the JMS Serializer to deserialize JSON into an existing object.</p>
  
  <p>Usually that would be useful for updating an existing object with new data that you have in a JSON format. Symfony's standard deserializer seems to offer that, but I can't seem to find anything about this with JMS. Have to use JMS though if I want the serializedName Annotation option.</p>
  
  <p>The "workaround" is to deserialize and then use Doctrine's EntityManager to merge, but that only works so well, and you can't easily discern which fields are updated if the JSON doesn't contain every single field.</p>