bpo-32634: fix write in email/generator.py

[reb00ter]

add_header when email dont have it

https://bugs.python.org/issue32634

[the-knights-who-say-ni]

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[matrixise]

Hi @reb00ter

Thank you for your contribution, it's great.

But for my part, I have few points.

1. there is no new tests with your fix
2. you have cleaned the code but it's not needed maybe you could just submit your fix and not the cleaning.

@bitdancer and @warsaw are the experts for the email part and I would like to have their opinion.

Thank you

[bitdancer]

Thanks for your work, but there is an existing PR for this almost ready to go (#1977). Also, FYI the style changes would have been rejected.