get_filename(), get_content_charset(): It's possible that the charset named in an RFC 2231-style header could be bogus or unknown to Python. In that case, we return the the text part of the parameter undecoded. However, in get_content_charset(), if that is not ascii, then it is an illegal charset and so we return failobj.
Test cases and a version bump are included.
Committing this to the Python 2.3 branch because I need to generate an email 2.5.6 release that contains these patches. I will port these fixes to Python 2.4 and 2.5 for email 3.x.