gh-121284: Fix email address header folding with parsed encoded-word (GH-122754)

Email generators using email.policy.default may convert an RFC 2047
encoded-word to unencoded form during header refolding. In a structured
header, this could allow 'specials' chars outside a quoted-string,
leading to invalid address headers and enabling spoofing. This change
ensures a parsed encoded-word that contains specials is kept as an
encoded-word while the header is refolded.

[Better fix from @bitdancer.]

---------

Co-authored-by: R David Murray <rdmurray@bitdance.com>
Co-authored-by: Petr Viktorin <encukou@gmail.com>