RequestListener.php

<?php

namespace AppBundle\EventListener;

use Symfony\Component\HttpKernel\Event\GetResponseEvent;

class RequestListener
{
    public function onKernelRequest(GetResponseEvent $event)
    {
        if (0 === strpos($event->getRequest()->headers->get('Content-Type'), 'application/json')) {
            $event->getRequest()->headers->set('Content-Type', 'application/json');
            $data = json_decode($event->getRequest()->getContent(), true);
            $event->getRequest()->request->replace(is_array($data) ? $data : array());
        }
    }
}

services.yml

  app.listener.request:
    class: AppBundle\EventListener\RequestListener
    tags:
      - { name: kernel.event_listener, event: kernel.request, method: onKernelRequest }