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".

deltachat-rpc-client/tests/test_something.py

@@ -660,14 +660,12 @@ def test_download_limit_chat_assignment(acfactory, tmp_path, n_accounts):
         contact = alice.create_contact(account)
         alice_group.add_contact(contact)
 
-    if n_accounts == 2:
-        bob_chat_alice = bob.create_chat(alice)
+    bob_chat_alice = bob.create_chat(alice)
     bob.set_config("download_limit", str(download_limit))
 
     alice_group.send_text("hi")
     snapshot = bob.wait_for_incoming_msg().get_snapshot()
     assert snapshot.text == "hi"
-    bob_group = snapshot.chat
 
     path = tmp_path / "large"
     path.write_bytes(os.urandom(download_limit + 1))
@@ -677,15 +675,7 @@ def test_download_limit_chat_assignment(acfactory, tmp_path, n_accounts):
         alice_group.send_file(str(path))
         snapshot = bob.wait_for_incoming_msg().get_snapshot()
         assert snapshot.download_state == DownloadState.AVAILABLE
-        if n_accounts > 2:
-            assert snapshot.chat == bob_group
-        else:
-            # Group contains only Alice and Bob,
-            # so partially downloaded messages are
-            # hard to distinguish from private replies to group messages.
-            #
-            # Message may be a private reply, so we assign it to 1:1 chat with Alice.
-            assert snapshot.chat == bob_chat_alice
+        assert snapshot.chat == bob_chat_alice
 
 
 def test_markseen_contact_request(acfactory):

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(())
 }