Webiny\Component\Mailer\Bridge\SwiftMailer\Message::addHeader PHP Method

    public function addHeader($name, $value, $params = null)
    {
        if (is_array($params)) {
            $this->message->getHeaders()->addParameterizedHeader($name, $value, $params);
        } else {
            $this->message->getHeaders()->addTextHeader($name, $value);
        }
        return $this;
    }

 /**
  * @dataProvider messageProvider
  *
  * @param Message $message
  *
  */
 public function testAddHeader($message)
 {
     $message->addHeader('test', 'value');
     $this->assertSame('value', $message->getHeader('test'));
     $this->assertArrayHasKey('test', $message->getHeaders());
 }