: fetch(set, attr)
: uid_fetch(set, attr)
      Sends a FETCH command to retrieve data associated with a message
      in the mailbox. the set parameter is a number or an array of
      numbers or a Range object. the number is a message sequence
      number (fetch) or a unique identifier (uid_fetch).
      The return value is an array of imap.rb"/Net::IMAP::FetchData.

      ex).
        p imap.fetch(6..8, "UID")
        #=> [#<Net::IMAP::FetchData seqno=6, attr={"UID"=>98}>, #<Net::IMAP::FetchData seqno=7, attr={"UID"=>99}>, #<Net::IMAP::FetchData seqno=8, attr={"UID"=>100}>]
        p imap.fetch(6, "BODY[HEADER.FIELDS (SUBJECT)]")
        #=> [#<Net::IMAP::FetchData seqno=6, attr={"BODY[HEADER.FIELDS (SUBJECT)]"=>"Subject: test\r\n\r\n"}>]
        data = imap.uid_fetch(98, ["RFC822.SIZE", "INTERNALDATE"])[0]
        p data.seqno
        #=> 6
        p data.attr["RFC822.SIZE"]
        #=> 611
        p data.attr["INTERNALDATE"]
        #=> "12-Oct-2000 22:40:59 +0900"
        p data.attr["UID"]
        #=> 98

