Mimeparser uses unprotected Date header

[link2xt]

We already protect the Date header against modification here:

core/src/mimefactory.rs

Lines 914 to 936 in e5a3eae

	} else if is_encrypted {
	protected_headers.push(header.clone());
	match header_name.as_str() {
	"subject" => {
	unprotected_headers.push((
	"Subject",
	mail_builder::headers::raw::Raw::new("[...]").into(),
	));
	}
	"date"
	| "in-reply-to"
	| "references"
	| "auto-submitted"
	| "chat-version"
	| "autocrypt-setup-message" => {
	unprotected_headers.push(header.clone());
	}
	_ => {
	// Other headers are removed from unprotected part.
	}
	}
	} else {

There is an open PR #6649 to protect the Date by replacing it with unix epoch in the outer header.

However, it cannot be merged because mimeparser uses unprotected timestamp for Autocrypt header here:

core/src/mimeparser.rs

Lines 348 to 353 in 7f55613

	if let Some(protected_aheader_value) = decrypted_mail
	.headers
	.get_header_value(HeaderDef::Autocrypt)
	{
	aheader_value = Some(protected_aheader_value);
	}

I also made a minor fix for signed-only messages: #6642
Fix for the Autocrypt issue is at #6669