fix: Only include one From: header in securejoin messages (#5917)

This fixes the bug that sometimes made QR scans fail.

The problem was:

When sorting headers into unprotected/hidden/protected, the From: header
was added twice for all messages: Once into unprotected_headers and once
into protected_headers. For messages that are `is_encrypted && verified
|| is_securejoin_message`, the display name is removed before pushing it
into unprotected_headers.

Later, duplicate headers are removed from unprotected_headers right
before prepending unprotected_headers to the message. But since the
unencrypted From: header got modified a bit when removing the display
name, it's not exactly the same anymore, so it's not removed from
unprotected_headers and consequently added again.

src/test_utils.rs

@@ -2,7 +2,7 @@
 //!
 //! This private module is only compiled for test runs.
 #![allow(clippy::indexing_slicing)]
-use std::collections::BTreeMap;
+use std::collections::{BTreeMap, HashSet};
 use std::fmt::Write;
 use std::ops::{Deref, DerefMut};
 use std::panic;
@@ -477,6 +477,36 @@ impl TestContext {
         update_msg_state(&self.ctx, msg_id, MessageState::OutDelivered)
             .await
             .expect("failed to update message state");
+
+        let payload_headers = payload.split("\r\n\r\n").next().unwrap().lines();
+        let payload_header_names: Vec<_> = payload_headers
+            .map(|h| h.split(':').next().unwrap())
+            .collect();
+
+        // Check that we are sending exactly one From, Subject, Date, To, Message-ID, and MIME-Version header:
+        for header in &[
+            "From",
+            "Subject",
+            "Date",
+            "To",
+            "Message-ID",
+            "MIME-Version",
+        ] {
+            assert_eq!(
+                payload_header_names.iter().filter(|h| *h == header).count(),
+                1,
+                "This sent email should contain the header {header} exactly 1 time:\n{payload}"
+            );
+        }
+        // Check that we aren't sending any header twice:
+        let mut hash_set = HashSet::new();
+        for header_name in payload_header_names {
+            assert!(
+                hash_set.insert(header_name),
+                "This sent email shouldn't contain the header {header_name} multiple times:\n{payload}"
+            );
+        }
+
         Some(SentMessage {
             payload,
             sender_msg_id: msg_id,