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.