Call setPartsToBody reliably to fix issue #5

[fheyer]

This is a fix for issue #5 (other mails are blank)

For all mail other than order, invoice, shipment, creditmemo (those with attachment) Mageplaza\EmailAttachments\Mail\Message::setPartsToBody is never called. That's why those mails don't have a body.

Longer explanation:
Problem is that line 54 from Mageplaza\EmailAttachments\Observer\AbstractEmail doesn't get called:

$this->mail->setTemplateVars($observer->getTransport());

So in turn Mageplaza\EmailAttachments\Model\Mail::$templateVars is null which makes Mageplaza\EmailAttachments\Model\MailEvent never call Mageplaza\EmailAttachments\Mail\Message::setPartsToBody (is only called if observer ran).

So the instance of Mageplaza\EmailAttachments\Mail\Message has an element in $parts but as setPartsToBody never gets called $zendMessage never gets a message body.

So in effect all other mails apart from those with attachment don't have a body.