Zend_Mail::clearMessageId PHP Method

    public function clearMessageId()
    {
        $this->_messageId = null;
        $this->clearHeader('Message-Id');
        return $this;
    }

 public function testClearMessageId()
 {
     $mail = new Zend_Mail();
     $mail->setMessageId();
     $mail->clearMessageId();
     $this->assertFalse(isset($mock->headers['Message-Id']));
 }