Set X-Message-Id with SentMessage::setMessageId

[jameswagoner]

WHAT

Propose using setMessageId() instead of headers for X-Message-Id

WHY

Event::listen(function (MessageSent $event) {
    // when sendgrid ID set with headers
    $sendgridId = $event->message->getHeaders()->getHeaderBody('x-sendgrid-message-id');

    // when sendgrid ID set with setMessageId()
    $sendgridId = $event->sent->getMessageId();
});

[parkourben99]

This would be a breaking change for anyone currently using this header.

[jameswagoner]

Totally thought of that later in the day. Haven't been back at my desk to add the header back in. I'll update my PR

[jameswagoner]

added header and test assertion back in