Function add_headers_to_encrypted_part 

fn add_headers_to_encrypted_part(
    message: MimePart<'static>,
    unprotected_headers: &[(&'static str, HeaderType<'static>)],
    hidden_headers: Vec<(&'static str, HeaderType<'static>)>,
    protected_headers: Vec<(&'static str, HeaderType<'static>)>,
    use_std_header_protection: bool,
) -> MimePart<'static>