fix: Add protected-headers directive to Content-Type of encrypted/signed MIME (#2302)

Add protected-headers="v1" directive to Content-Type of an encrypted/signed MIME so that other MUAs
like Thunderbird display the true message Subject instead of "...".

src/test_utils.rs

@@ -108,9 +108,15 @@ impl TestContextManager {
     /// - Let one TestContext send a message
     /// - Let the other TestContext receive it and accept the chat
     /// - Assert that the message arrived
-    pub async fn send_recv_accept(&self, from: &TestContext, to: &TestContext, msg: &str) {
+    pub async fn send_recv_accept(
+        &self,
+        from: &TestContext,
+        to: &TestContext,
+        msg: &str,
+    ) -> Message {
         let received_msg = self.send_recv(from, to, msg).await;
         received_msg.chat_id.accept(to).await.unwrap();
+        received_msg
     }
 
     /// - Let one TestContext send a message