Support Sendgrid's hosted templates (D8 edition)

Referencing this issue here (https://www.drupal.org/node/2696753).

I currently have this module installed, and noticed that the patch that was merged in the above issue does not work in the D8 version.

After digging into it, I'm noticing there is the file sendgrid.mail.inc, but also a Mail Plugin: SendgridMail.php. Seems like sendgrid.mail.inc is deprecated? Or maybe this just exists for some backwards compatibility? Either way, the logic never made it's way into SendgridMail.php.

My solution was to add the same logic from the referenced issue into SendgridMail.php, see uploaded patch file. Confirmed this works on a Drupal 8.3x install.

This is my first issue and contribution I've created on drupal.org, please let me know if I've left out some critical information, or done something dumb.

Cheers,
Liam