Fixes (mysterious, to the end user) UnicodeErrors when using utf-8 as
the charset and unicode as the _text argument. Also makes the way in
which unicode gets encoded to quoted printable for other charsets more
sane (it only worked by accident previously). The _payload now is encoded
to the charset.output_charset if it is unicode.
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.