get_param(): Update the docstring to explain how CHARSET and LANGUAGE

can be None, and what to do in that situation.

get_filename(), get_boundary(), get_content_charset(): Make sure these
handle RFC 2231 headers without a CHARSET field.

Backport candidate (as was the Utils.py 1.25 change) to both Python
2.3.1 and 2.2.4 -- will do momentarily.