FieldStorage.__init__(): if there is no content-type header, use

FieldStorage.__init__(): if there is no content-type header, use
text/plain for inner parts, but application/x-www-form-urlencoded
for outer parts.  Honor any existing content-type header.

Lower down, if the content-type header is something we don't
understand (say because it there was a typo in the header coming from
the client), default to text/plain for inner parts, but
application/x-www-form-urlencoded for outer parts.