Type error in Message->getPriority()

[bretislavstepan]

getPriority has return type ?int but getHeader returns mixed|null (in my case numeric string '3').

mail/src/Mail/Message.php

Line 180 in dfd7576

return $this->getHeader('X-Priority');

Can I send a PR with a fix?

$priority = $this->getHeader('X-Priority');
return is_numeric($priority) ? (int) $priority : null;