Magento 2 – Override Model Function

I want to override one of the function in Magento core model which is located in

vendor\magento\module-sales\Model\Order\Email\Sender\OrderSender.php

<?php
/**
 * Copyright © 2016 Magento. All rights reserved.
 * See COPYING.txt for license details.
 */
namespace Magento\Sales\Model\Order\Email\Sender;

use Magento\Payment\Helper\Data as PaymentHelper;
use Magento\Sales\Model\Order;
use Magento\Sales\Model\Order\Email\Container\OrderIdentity;
use Magento\Sales\Model\Order\Email\Container\Template;
use Magento\Sales\Model\Order\Email\Sender;
use Magento\Sales\Model\ResourceModel\Order as OrderResource;
use Magento\Sales\Model\Order\Address\Renderer;
use Magento\Framework\Event\ManagerInterface;

/**
 * Class OrderSender
 * @SuppressWarnings(PHPMD.CouplingBetweenObjects)
 */
class OrderSender extends Sender
{
  protected function prepareTemplate(Order $order)
    {
        $transport = [
            'order' => $order, 
            'billing' => $order->getBillingAddress(),
            'payment_html' => $this->getPaymentHtml($order),
            'store' => $order->getStore(),
            'formattedShippingAddress' => $this->getFormattedShippingAddress($order),
            'formattedBillingAddress' => $this->getFormattedBillingAddress($order),
        ];

        $transport = new \Magento\Framework\DataObject($transport);

        $this->eventManager->dispatch(
            'email_order_set_template_vars_before',
            ['sender' => $this, 'transport' => $transport]
        );

        $this->templateContainer->setTemplateVars($transport->getData());

        parent::prepareTemplate($order);
    }
}

I just want to add additional data to $transport array variable from my custom extension, but it seems not working because when I call the additional data it didn't print anything, here's what i've done:

etc/di.xml

<?xml version="1.0" ?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <preference for="Magento\Sales\Model\Order\Email\Sender\OrderSender" type="Namespace\Module\Model\Order\Email\Sender\OrderSender"/>
</config>

Namespace\Module\Model\Order\Email\Sender\OrderSender.php

<?php
/**
 * Copyright © 2016 Magento. All rights reserved.
 * See COPYING.txt for license details.
 */
namespace Namespace\Module\Model\Order\Email\Sender;

use Magento\Payment\Helper\Data as PaymentHelper;
use Magento\Sales\Model\Order;
use Magento\Sales\Model\Order\Email\Container\OrderIdentity;
use Magento\Sales\Model\Order\Email\Container\Template;
use Magento\Sales\Model\Order\Email\Sender;
use Magento\Sales\Model\ResourceModel\Order as OrderResource;
use Magento\Sales\Model\Order\Address\Renderer;
use Magento\Framework\Event\ManagerInterface;

/**
 * Class OrderSender
 * @SuppressWarnings(PHPMD.CouplingBetweenObjects)
 */
class OrderSender extends \Magento\Sales\Model\Order\Email\Sender\OrderSender
{


    public function prepareTemplate(Order $order)
    {

        $transport = [
            'order' => $order,
            'billing' => $order->getBillingAddress(),
            'payment_html' => $this->getPaymentHtml($order),
            'test' => '<p> Yoo Yeezy </p>',
            'store' => $order->getStore(),
            'formattedShippingAddress' => $this->getFormattedShippingAddress($order),
            'formattedBillingAddress' => $this->getFormattedBillingAddress($order),
        ];
        $transport = new \Magento\Framework\DataObject($transport);

        $this->eventManager->dispatch(
            'email_order_set_template_vars_before',
            ['sender' => $this, 'transport' => $transport]
        );

        $this->templateContainer->setTemplateVars($transport->getData());

        parent::prepareTemplate($order);
    }


}