Magento 2.2 Compatibility

[ryantfowler]

M2.2 introduced \Magento\Email\Model\Transport whereas the current version of this extension has a di preference for \Magento\Framework\Mail\Transport. A quick-n-dirty fix is to change this for the di.xml preference ... but this breaks backwards compatibility. I suppose either rewriting some of the module to instead implement a Plugin which targets the \Magento\Framework\Mail\TransportInterface or something similar is in order to attempt to keep backwards compatibility ... unless the maintainer(s) would like to create a new release that supports M2.2.x?

In case there are no objections for supporting 2.2 moving forward, there's this PR

[sandip5678]

Used for M2.2 but not showing headers in the email original message