Make Sales' emails $transport changeable from an observer

[obuchowski]

In the commit
44791d5#diff-f625923e436a1fdf17c94533d06a45fb
$transport variable type was changed from array to \Magento\Framework\DataObject. in the
Magento/Sales/Model/Order/Email/Sender/OrderSender class

As the result OrderSender's $transport became changeable from an observer.
But OrderCommentSender, InvoiceSender and others had been missed.
This PR fixes it.

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)

[miguelbalparda]

Hi @EObukhovsky! Thanks for your contribution, I'll check this and get back to you if we have questions!

[obuchowski]

@miguelbalparda thanks for the quick response!

[miguelbalparda]

@EObukhovsky this seems to be a feature instead of a bug but I still see a lot of value in the PR. It might take some extra time to process but I'll do my best have this merged. Thanks again!

[miguelbalparda]

After reviewing this in detail, it seems the original commit you mentioned introduced a change in the code and now the event variable transport is an object instead of an array. That might be a problem for backward compatibility, source here.
This might be fixed by adding a getData() in the code to send an array or by adding an extra param, we will discuss this internally and get back to you with the next steps.
Thanks again!

[obuchowski]

@miguelbalparda good. A one more note from me:
The original problem that brought me here was a need to add an additional variable into sales emails. At the moment I forced to use workarounds like $order->setData() in observer for these purposes.
And I believe you guys find a solution.