Coerce Gmail labels to unicode if necessary.

Turns out that if you have label "22", imapclient will parse the response
< * 82 FETCH (X-GM-LABELS ("\\Inbox" 5) UID 395 MODSEQ (40256))
into
{395: {u'X-GM-LABELS': (u'\\Inbox', 5), u'SEQ': 82, u'MODSEQ': (40256,)}}
so we have to turn that int into a unicode string.