feat: Don't copy References and In-Reply-To to outer headers

This implements the suggestion from
https://www.rfc-editor.org/rfc/rfc9788.html#name-offering-more-ambitious-hea of "Header Protection
for Cryptographically Protected Email".

Author: iequidoo

src/mimefactory.rs

@@ -1062,7 +1062,7 @@ impl MimeFactory {
                             mail_builder::headers::raw::Raw::new("[...]").into(),
                         ));
                     }
-                    "in-reply-to" | "references" | "autocrypt-setup-message" => {
+                    "autocrypt-setup-message" => {
                         unprotected_headers.push(header.clone());
                     }
                     _ => {

src/receive_imf/receive_imf_tests.rs

@@ -4849,14 +4849,15 @@ async fn test_prefer_references_to_downloaded_msgs() -> Result<()> {
     let received = bob.recv_msg(&sent).await;
     assert_eq!(received.download_state, DownloadState::Available);
     assert_ne!(received.chat_id, bob_chat_id);
-    assert_eq!(received.chat_id, bob.get_chat(alice).await.id);
+    let bob_alice_chat_id = bob.get_chat(alice).await.id;
+    assert_eq!(received.chat_id, bob_alice_chat_id);
 
     let mut msg = Message::new(Viewtype::File);
     msg.set_file_from_bytes(alice, "file", file_bytes, None)?;
     let sent = alice.send_msg(alice_chat_id, &mut msg).await;
     let received = bob.recv_msg(&sent).await;
     assert_eq!(received.download_state, DownloadState::Available);
-    assert_eq!(received.chat_id, bob_chat_id);
+    assert_eq!(received.chat_id, bob_alice_chat_id);
 
     Ok(())
 }