fix: return correct header keys for each integration

lambda-http/src/response.rs

@@ -69,8 +69,8 @@ impl LambdaResponse {
                 body,
                 is_base64_encoded,
                 status_code: status_code as i64,
-                // explicitly empty, as API gateway does not properly merge headers and
-                // multi-value-headers, resulting in duplicate headers
+                // Explicitly empty, as API gateway v1 will merge "headers" and
+                // "multi_value_headers" fields together resulting in duplicate response headers.
                 headers: HeaderMap::new(),
                 multi_value_headers: headers,
             }),
@@ -93,21 +93,20 @@ impl LambdaResponse {
                     is_base64_encoded,
                     status_code: status_code as i64,
                     cookies,
-                    // explicitly empty, as API gateway does not properly merge headers and
-                    // multi-value-headers, resulting in duplicate headers
-                    headers: HeaderMap::new(),
-                    multi_value_headers: headers,
+                    // API gateway v2 doesn't have multiValueHeaders field. Duplicate headers are
+                    // combined with commas and included in the headers field.
+                    headers: headers,
+                    multi_value_headers: HeaderMap::new(),
                 })
             }
             #[cfg(feature = "alb")]
             RequestOrigin::Alb => LambdaResponse::Alb(AlbTargetGroupResponse {
                 body,
                 status_code: status_code as i64,
                 is_base64_encoded,
-                // ALB responses are used for ALB integrations as well as
-                // Lambda Function URLs. The former uses the `multi_value_headers` field,
-                // while the later uses the `headers` field. We need to return
-                // both fields to ensure both integrations work correctly.
+                // ALB responses are used for ALB integration, which can be configured to use
+                // either headers or multi-value headers field. We need to return both fields to
+                // ensure both configuration work correctly.
                 headers: headers.clone(),
                 multi_value_headers: headers,
                 status_description: Some(format!(
@@ -475,7 +474,7 @@ mod tests {
         let json = serde_json::to_string(&response).expect("failed to serialize to json");
         assert_eq!(
             json,
-            r#"{"statusCode":200,"headers":{},"multiValueHeaders":{"content-encoding":["gzip"]},"body":"MDAwMDAw","isBase64Encoded":true,"cookies":[]}"#
+            r#"{"statusCode":200,"headers":{"content-encoding":"gzip"},"multiValueHeaders":{},"body":"MDAwMDAw","isBase64Encoded":true,"cookies":[]}"#
         )
     }
 
@@ -493,7 +492,7 @@ mod tests {
         let json = serde_json::to_string(&response).expect("failed to serialize to json");
         assert_eq!(
             json,
-            r#"{"statusCode":200,"headers":{},"multiValueHeaders":{"content-type":["application/json"]},"body":"000000","isBase64Encoded":false,"cookies":[]}"#
+            r#"{"statusCode":200,"headers":{"content-type":"application/json"},"multiValueHeaders":{},"body":"000000","isBase64Encoded":false,"cookies":[]}"#
         )
     }
 
@@ -511,7 +510,7 @@ mod tests {
         let json = serde_json::to_string(&response).expect("failed to serialize to json");
         assert_eq!(
             json,
-            r#"{"statusCode":200,"headers":{},"multiValueHeaders":{"content-type":["application/json; charset=utf-16"]},"body":"〰〰〰","isBase64Encoded":false,"cookies":[]}"#
+            r#"{"statusCode":200,"headers":{"content-type":"application/json; charset=utf-16"},"multiValueHeaders":{},"body":"〰〰〰","isBase64Encoded":false,"cookies":[]}"#
         )
     }
 
@@ -529,7 +528,7 @@ mod tests {
         let json = serde_json::to_string(&response).expect("failed to serialize to json");
         assert_eq!(
             json,
-            r#"{"statusCode":200,"headers":{},"multiValueHeaders":{"content-type":["application/graphql-response+json; charset=utf-16"]},"body":"〰〰〰","isBase64Encoded":false,"cookies":[]}"#
+            r#"{"statusCode":200,"headers":{"content-type":"application/graphql-response+json; charset=utf-16"},"multiValueHeaders":{},"body":"〰〰〰","isBase64Encoded":false,"cookies":[]}"#
         )
     }