Removed default charset of UTF-8. User may specify their own charset

[glennzw]

This IMAP library fails for Microsoft Exchange servers because of the hardcoded UTF-8 charset:

imap: unexpected completion status (\"NHGVU5 NO [BADCHARSET (US-ASCII)] The specified charset is not supported.\")"

I've removed the default charset, and users may rather specify their own charset in the []imap.Field. e.g:

specs := []imap.Field{"CHARSET", "US-ASCII", "UNSEEN"}

I tested with a few different IMAP servers and all were happy with no charset set.