fix(http1): use append for repeat trailer values in encoder (#4118)

The chunked trailer encoder built allowed_trailers with HeaderMap::insert,
which overwrites prior values for the same name. When a user sets the same
trailer field multiple times, only the last value was written to the wire.

This mirrors the decode-side fix (#4107): use append so duplicate trailer
values are preserved on encode, matching how regular headers are emitted.

Author: mahirhir

src/proto/h1/encode.rs

@@ -181,7 +181,7 @@ impl Encoder {
 
                     if allowed_set.contains(name) {
                         if is_valid_trailer_field(name) {
-                            allowed_trailers.insert(name, value);
+                            allowed_trailers.append(name, value);
                         } else {
                             debug!("trailer field is not valid: {}", &name);
                         }
@@ -567,6 +567,32 @@ mod tests {
         );
     }
 
+    #[test]
+    fn chunked_with_duplicate_trailer_values() {
+        let encoder = Encoder::chunked();
+        let trailers = vec![HeaderName::from_static("chunky-trailer")];
+        let encoder = encoder.into_chunked_with_trailing_fields(trailers);
+
+        let mut headers = HeaderMap::new();
+        headers.append(
+            HeaderName::from_static("chunky-trailer"),
+            HeaderValue::from_static("first"),
+        );
+        headers.append(
+            HeaderName::from_static("chunky-trailer"),
+            HeaderValue::from_static("second"),
+        );
+
+        let buf1 = encoder.encode_trailers::<&[u8]>(headers, false).unwrap();
+
+        let mut dst = Vec::new();
+        dst.put(buf1);
+        assert_eq!(
+            dst,
+            b"0\r\nchunky-trailer: first\r\nchunky-trailer: second\r\n\r\n"
+        );
+    }
+
     #[test]
     fn chunked_with_no_trailer_header() {
         let encoder = Encoder::chunked();