imaplib2: fix handling of zero-sized IMAP objects

self._expecting_data was used both as the expected data length and the
flag that we expect some data.  This obviously fails when advertized
data length is zero, so self._expecting_data_len was introduced to
hold the length of the expected data and self._expecting_data was left
as the flag that we expect the data to come.

GitHub issue: https://github.com/OfflineIMAP/offlineimap/issues/15
Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>