Allow 0 as a UIDVALIDITY resp-code value from the server
Allow 0 as a UIDVALIDITY resp-code value from the server

Fixes issue #150

matt2003/MailKit,mganss/MailKit,dsbenghe/MailKit,jamie-dainton/MailKit,eino-makitalo/MailKit,jstedfast/MailKit,FubarDevelopment/MailKit,sstraus/MailKit,nachocove/MailKit