Fix base64 encoding in alt body with long lines
Fix base64 encoding in alt body with long lines

Extend body base64 fix from https://github.com/PHPMailer/PHPMailer/commit/9269a656ca2dca1ab2653c4abe7c0f796f31af33 to alt body.
Synchro/PHPMailer,Dubz/PHPMailer,PHPMailer/PHPMailer,bertpoort/PHPMailer,iFixit/PHPMailer,sherryl4george/PHPMailer,jimjag/PHPMailer,PHPMailer/PHPMailer,ravisorg/PHPMailer,iFixit/PHPMailer,nawawi/PHPMailer,ravisorg/PHPMailer,PunchyRascal/PHPMailer,pingyen/PHPMailer,sherryl4george/PHPMailer,namepros/PHPMailer,martinvd/PHPMailer,jimjag/PHPMailer,JesusTheHun/PHPMailer,pingyen/PHPMailer,Synchro/PHPMailer,PunchyRascal/PHPMailer,nawawi/PHPMailer,JesusTheHun/PHPMailer,Dubz/PHPMailer,martinvd/PHPMailer,namepros/PHPMailer,bertpoort/PHPMailer