Bug reported by Gaël Le Mignot: in a sample case he encountered,
"description" was a unicode object, causing encoding failure in packet
construction.
Python "str" is used in ZODB FileStorage code (see format.py), so just do
the same.

git-svn-id: https://svn.erp5.org/repos/neo/trunk@2167 71dcc9de-d417-0410-9af5-da40c76e7ee4