This should fail for custom fold() implementations that aren't careful
about newlines.

Per RFC 2047:

> [...] these encoding schemes allow the
> encoding of arbitrary octet values, mail readers that implement this
> decoding should also ensure that display of the decoded data on the
> recipient's terminal will not cause unwanted side-effects

It seems that the "quoted-word" scheme is a valid way to include
a newline character in a header value, just like we already allow
undecodable bytes or control characters.
They do need to be properly quoted when serialized to text, though.

---

Credit for an earlier attempt:

Co-Authored-By: Bas Bloemsaat <[email protected]>

I'm not touching other instances in this file, since this PR might
be backported to very old versions.

Co-authored-by: Serhiy Storchaka <[email protected]>