gh-153822: Recreate the headers from raw form when part of signed data

[MichalKarol]

When Message is part of multipart/signed, the headers are geneated from the raw header value to fully reproduce folding.

• Issue: Email's as_bytes() method unfolds a protected header in a pgp signed message part breaking the signature. #153822

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[python-cla-bot]

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[bedevere-app]

Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.

[bedevere-app]

Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.

[zware]

@MichalKarol I'm not qualified to actually review this, but looking at the issue I think this is worth a NEWS entry if this fixes the issue. (As Chris mentioned in person, at worst the proper reviewer will just ask that it be removed if I'm wrong and it does turn out to be superfluous.)