[3.9] pythongh-121284: Fix email address header folding with parsed encoded-word (pythonGH-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.]

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(cherry picked from commit 295b53d)

Co-authored-by: Mike Edmunds <[email protected]>
Co-authored-by: R David Murray <[email protected]>
Co-authored-by: Petr Viktorin <[email protected]>

Author: 3 people