milter - fix for py3 in noReply, force_bString converts types other than string/unicode
Also changes behavior of force_bString which was returning objects other than unicode unchanged. This is a bit inconsistent with the name of the function which should enforce an encoded string.
Following the concept of force_uString which handles any object which can be converted to string
the expected output of force_bString(550)
is b'550'
This fixes the Py3 issue in libmilter with a conversion error in setReply