Webiny\Component\Rest\Response\CallbackResult::setHeaderResponse PHP Method

    public function setHeaderResponse($statusCode, $message = '')
    {
        $this->statusCode = $statusCode;
        $this->message = $message;
        if ($message == '') {
            $this->message = self::$defaultMessages[$statusCode];
        }
        if (empty($this->message)) {
            throw new RestException('Invalid http response status code: ' . $statusCode);
        }
        return $this;
    }

 public function testGetOutput()
 {
     $cr = new CallbackResult();
     $cr->setHeaderResponse(403, 'TestMessage')->setEnvToDevelopment()->setData('test content');
     $response = $cr->getOutput();
     $this->assertSame(['data' => 'test content'], $response);
 }