[Imap-protocol] NILs

I realized today I had several bugs related to NILs. I’ll add these to imaptest, but thought I’d mention here also in case others have similar issues:

1. My understanding from RFC 3501 is that NILs are case-insensitive, because of:

        Note: [ABNF] rules MUST be followed strictly; in
        particular:

        (1) Except as noted otherwise, all alphabetic characters
        are case-insensitive.  The use of upper or lower case
        characters to define token strings is for editorial clarity
        only.  Implementations MUST accept these strings in a
        case-insensitive fashion.

I was checking only for uppercase NILs. Then again there weren't actually any IMAP command parameters where NIL was allowed before I implemented SETMETADATA today.

2. NIL is a valid atom in astrings, and must be treated the same as “NIL”:

        Note: NIL is never used for any data item which takes the
        form of an atom.  For example, a mailbox name of "NIL" is a
        mailbox named NIL as opposed to a non-existent mailbox
        name.  This is because mailbox uses "astring" syntax which
        is an atom or a string.

And of course this needs to work case-sensitively when used in mailbox names, so NIL != nil.

3. When sending NIL (case-insensitive) astring to clients, it’s better to send it quoted. Clients are much less likely to implement 2 correctly, and I’m sure in some cases my code is calling write_astring() when it’s really nstring in ABNF.

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