add charset to default content-type header if STDOUT is in UTF-8

If output has utf8 layer enabled, it makes only sense to mark it as such
to clients.  Only available with PerlIO (as of Perl 5.8 afaict), but not
fatal (assume non-utf8) if it doesn't work.

Charset is only added at first read or change of the content-type value,
so it's not hardcoded to site defaults.  Requires an extra object
variable unfortunately, but the only way to make it useful (no page
runtime accomodation wouldn't give much advantage).