def getquotaroot(mailbox)
      synchronize do
        send_command("GETQUOTAROOT", mailbox)
        result = []
        result.concat(@responses.delete("QUOTAROOT"))
        result.concat(@responses.delete("QUOTA"))
        return result
      end
    end
Sends the GETQUOTAROOT command along with specified +mailbox+.
This command is generally available to both admin and user.
If mailbox exists, returns an array containing objects of
Net::IMAP::MailboxQuotaRoot and Net::IMAP::MailboxQuota.