Add transportMessageHeaders

[markb-trustifi]

Please add new property with raw unparsed headers - transportMessageHeaders.
Right now, because the MailParser instance is not exported from the nodemailer library, the raw headers can be received only by updating the simple-parser.js code:

parser.on('headers', headers => {
        mail.headers = headers;
        mail.headerLines = parser.headerLines;
        mail.transportMessageHeaders = parser.splitter.node.headers.headers.toString();
});