understand and test IMAP String encoding

[weissi]

Currently, we’re just handing through the raw bytes but IMAP does have some charset support and also there’s some UTF-7 (not a typo) stuff in the spec.

We need to understand which things in IMAP are fundamentally Strings (and therefore need decoding with the correct charset) and which things are raw bytes.

[weissi]

Note (thanks @danieleggert) that we MUST NOT decode MailboxName into a String or at least we need to preserve the ByteBuffer and surface it to the user. The reason (again thanks @danieleggert) is that the actual identifier for a mailbox is its names as raw bytes, so IIUC, in theory we could have two mailboxes called "Föö" one encoded as UTF7, the other one as UTF8. Doesn't make much sense of course but in theory it exists.