问如何在Laravel 5.1中强制FormRequest返回json？

EN

//Illuminate\Foundation\Http\FormRequest class
/**
 * Get the proper failed validation response for the request.
 *
 * @param  array  $errors
 * @return \Symfony\Component\HttpFoundation\Response
 */
public function response(array $errors)
{
    if ($this->ajax() || $this->wantsJson()) {
        return new JsonResponse($errors, 422);
    }

    return $this->redirector->to($this->getRedirectUrl())
                                    ->withInput($this->except($this->dontFlash))
                                    ->withErrors($errors, $this->errorBag);
}

forceJsonResponse || $this->ajax() || $this->wantsJson()) {
            return new JsonResponse($errors, 422);
        }

        return $this->redirector->to($this->getRedirectUrl())
            ->withInput($this->except($this->dontFlash))
            ->withErrors($errors, $this->errorBag);
    }
}

'required|unique:posts|max:255',
            'body' => 'required',
        ];
    }
}

namespace Laravel5Cg\Http\Middleware;

use Closure;
use Symfony\Component\HttpFoundation\HeaderBag;

class AddJsonAcceptHeader
{
    /**
     * Add Json HTTP_ACCEPT header for an incoming request.
     *
     * @param  \Illuminate\Http\Request  $request
     * @param  \Closure  $next
     * @return mixed
     */
    public function handle($request, Closure $next)
    {
        $request->server->set('HTTP_ACCEPT', 'application/json');
        $request->headers = new HeaderBag($request->server->getHeaders());
        return $next($request);
    }
}