Allow a UIDVALIDITY value of 0 for the APPENDUID resp-code as well
Allow a UIDVALIDITY value of 0 for the APPENDUID resp-code as well

dsbenghe/MailKit,eino-makitalo/MailKit,jstedfast/MailKit,jamie-dainton/MailKit,sstraus/MailKit,nachocove/MailKit