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>

Changelog.rst

@@ -28,6 +28,7 @@ WIP (add new stuff for the next release)
   local repository for the account being processed.
 * [regression] pass folder names to the foldersort function,
   revert the documented behaviour
+* Fix handling of zero-sized IMAP data items (GitHub#15).
 
 OfflineIMAP v6.5.5-rc1 (2012-09-05)
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