Allow the UIDVALIDITY argument to the COPYUID resp-code to be 0
Allow the UIDVALIDITY argument to the COPYUID resp-code to be 0

A UIDVALIDITY value of 0 is illegal, but this patch allows clients to
continue interoperating with the broken server.

Fixes issue #240

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