Handle character encoding when a proc is used as the response body

[cbaines]

In the web server. This means that characters not representable in ISO-8859-1 won't get transformed to a ?, if the charset is set to something that supports them, e.g. utf8.

[cbaines]

Not quite sure why, but I think this only works with the chunked-output-port in Guile 3.0.10.

[civodul]

Hey @cbaines,

Indeed, the new test fails for me when run under guix shell -CP from within a Fibers checkout (it’s using 3.0.9 here).

Could you take a look?

(Also, it’d be great if you could write ChangeLog-style commit log as is usually done here.)