acceptable to Python 2.4's time.strftime().  This fix mirrors the behavior in
email 3.0.  That field is documented as being "not useable" so it might as
well not be buggy too <wink>.

Add a test for this behavior and update a few tests that were expecting a 0 in
this field.  After committing I will run the entire Python 2.3 test suite to
ensure this doesn't break any Python tests.