Serializing Doctrine objects to proper JSON content type using ResponseTrait

Dear all, 

I'm trying to return Doctrine serialized objects (thanks to JMS Serializer) as JSON.
I am extending the ResourceController (and then using ResponseTrait)

The problem is that the returned headers are of type "text/html" instead of "application/json" when i use Respond.
If I use the more classic Response->setJSON, then the header is fine.

If I well understand, Respond will always consider returning text/html when receiving a string as argument. 
How to get around this? 

PHP Code:

class Employees extends \CodeIgniter\RESTful\ResourceController
{
    public function index()
    {
        $serializer = SerializerBuilder::create()->build();
        $res = $serializer->serialize(getEmployeeRepo()->findAll(), 'json');

        //return $this->response->setJSON($res); //working well: application/json is returned
        return $this->respond($res); //faulty: text/html is returned
    }
} 


Thank you

Well, I think I get a workaround.

Using Serializer.toArray(); instead of Serializer.serialize(); will do the trick.
       

PHP Code:

$res = $serializer->toArray(getEmployeeRepo()->findAll());
return $this->respond($res); //application/json is returned 


$this->respond($res) sets the Content-Type: text/html header because $res is a string.