Forward port some fixes that were in email 2.5 but for some reason didn't
    make it into email 4.0.  Specifically, in Message.get_content_charset(),
    handle RFC 2231 headers that contain an encoding not known to Python, or a
    character in the data that isn't in the charset encoding.  Also forward
    port the appropriate unit tests.

Also, this resolves SF bug #1414018.