Allow namespaced XML element discriminator

[goetas]

Allows to :

use JMS\Serializer\Annotation as Serializer;
/**
 * @Serializer\Discriminator(field = "type", map = {
 *    "tiger": "Ns\Tiger"
 * })
 * @Serializer\XmlDiscriminator(namespace="http://example.com/", cdata=false)
 * @Serializer\XmlNamespace(prefix="foo", uri="http://example.com/")
 */
class Animal
{

}


$serializer->serialize(new Tiger());

<result xmlns:foo="http://example.com/">
  <foo:type>tiger</foo:type>
</result>