Add support for [RFC2387 child element content-type hint](https://tools.ietf.org/html/rfc2387#section-3.1) in multipart/related structures
Add support for [RFC2387 child element content-type hint](https://tools.ietf.org/html/rfc2387#section-3.1) in multipart/related structures

nawawi/PHPMailer,nawawi/PHPMailer,jimjag/PHPMailer,PHPMailer/PHPMailer,PHPMailer/PHPMailer,jimjag/PHPMailer