10210: Transport variable can not be altered in email_invoice_set_template_vars_before Event (backport MAGETWO-69482 to 2.1).

[RomaKis]

Backport MAGETWO-69482.

Fixed Issues (if relevant)

1. Transport variable can not be altered in email_invoice_set_template_vars_before Event #10210: Transport variable can not be altered in email_invoice_set_template_vars_before Event (backport MAGETWO-69482 to 2.1).

Manual testing scenarios

1. Declare the event

<event name="email_invoice_set_template_vars_before">
        <observer name="mod_mail_variables_of_Invoice" instance="Vendor\Module\Observer\ChangePaymentHtml" />
    </event>

2. Alter variable payment_html in Vendor\Module\Observer\ChangePaymentHtml like

<?php

namespace Venoor\Module\Observer;

use Magento\Framework\Event\ObserverInterface;
use Magento\Framework\Event\Observer;
use Magento\Framework\App\Request\DataPersistorInterface;
use Magento\Framework\App\ObjectManager;
use Vendor\Module\Helper\Data as PaymentHelper;
use Magento\Sales\Model\Order;

class ChangePaymentHtml implements ObserverInterface
{
    public function execute(\Magento\Framework\Event\Observer $observer)
    {
         $transport = $observer->getEvent()->getData('transportObject');

        $transport['payment_html'] = '<div>SOME PAYMENT INFO</div>';
        $observer->setData('transportObject', $transport);
    }
}

3. Create order -> create Invoice.
4. Open invoice.
5. Press Send email.
6. Open email. Payment method should be "SOME PAYMENT INFO" .

Expected result

1. Email should be modified and new or changed variables should be output in email.

Actual result

1. Email is unchanged, and $transport variable seems not to be changed.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)