Please review the attached patch.  It changes the default charset
for set_content_type() to None.  If HTTPResponse.charset is None
then the charset parameter of the Content-Type header will not be
specified.  If you think it is less surprising, we can make the
'charset' argument to set_content_type() a required parameter.

The other change is to add quixote.DEFAULT_HTTP_CHARSET and
quixote.DEFAULT_STR_CHARSET.  DEFAULT_HTTP_CHARSET is nice because
it provides a single place to set the default encoding that Quixote
uses.  DEFAULT_STR_CHARSET is for insane  people who think
that str instances should contain encoded character data.  Also,
people who use unicode can set it to 'ascii' and ensure that
http_response._decode_string() always returns unicode strings.

  Neil