Fixing some RFC 2231 related issues as reported in the Spambayes

Fixing some RFC 2231 related issues as reported in the Spambayes
project, and with assistance from Oleg Broytmann.  Specifically,

get_param(), get_params(): Document that these methods may return
parameter values that are either strings, or 3-tuples in the case of
RFC 2231 encoded parameters.  The application should be prepared to
deal with such return values.

get_boundary(): Be prepared to deal with RFC 2231 encoded boundary
parameters.  It makes little sense to have boundaries that are
anything but ascii, so if we get back a 3-tuple from get_param() we
will decode it into ascii and let any failures percolate up.

get_content_charset(): New method which treats the charset parameter
just like the boundary parameter in get_boundary().  Note that
"get_charset()" was already taken to return the default Charset
object.

get_charsets(): Rewrite to use get_content_charset().