text/plain form submission seems broken encoding-wise

[annevk]

While looking at #3223 I noticed that text/plain never actually invokes the Encoding Standard itself. The text/plain algorithm has two callers and one of those does call the Encoding Standard, but the other one always UTF-8 encodes, which would mismatch with any charset fields.

It doesn't seem quite right. And if it is right we should have a note there about the strangeness.

It might be worth fixing #2223 first.

[annevk]

This seems somewhat reasonable as "mail as body" ends up creating a URL. The integration with the URL Standard needs updating though.