Encrypted messages with attachments don't have protected-headers="v1" in the header

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This line does not work for multipart/mixed parts:
https://github.com/deltachat/deltachat-core-rust/blob/36aef6499d0c2e2787cc8ee2bd798a6c9a7400a0/src/mimefactory.rs#L591

The result is still Content-Type: multipart/mixed; boundary="some-random-string", so Thunderbird does not look for protected headers and displays message Subject as ....

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The source of the problem is here:
https://github.com/deltachat/rust-email/blob/25702df99254d059483b41417cd80696a258df8e/src/message.rs#L189

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Marking it as a good first issue because it is clear what the result should be, but I am not sure it is actually easy to implement.