Modified ImapClient.GetFolder(SpecialFolder) to throw NotSupportedException
Modified ImapClient.GetFolder(SpecialFolder) to throw NotSupportedException

If the IMAP server doesn't support SPECIAL-USE or XLIST, throw
NotSupportedException to make it more obvious to developers
that not all IMAP servers support this feature.

jstedfast/MailKit,dsbenghe/MailKit,jamie-dainton/MailKit,nachocove/MailKit