Date header cannot be taken from protected headers

[link2xt]

While making PR #5715 I discovered that Date header cannot be protected. If it is moved into protected part and removed from unprotected IMF header, mimeparser ignores it because it extracts the Date header before decrypting the message:
https://github.com/deltachat/deltachat-core-rust/blob/d40ec88b9458ea425b6a1ac04549daa5cae32554/src/mimeparser.rs#L223-L229

This receiver side should be fixed and the fix should be released before we start protecting the Date header. Even if we decide that we don't want it (because of possibility of decryption failures and non-fully-downloaded message), receiver should support it.