public function testSetCallbackSerializationContextWithNotSerializeNull()
 {
     $this->builder->setDefaultSerializationContextFactory(function () {
         return SerializationContext::create()->setSerializeNull(false);
     });
     $serializer = $this->builder->build();
     $result = $serializer->serialize(array('value' => null, 'not_null' => 'ok'), 'json');
     $this->assertEquals('{"not_null":"ok"}', $result);
 }