fix: return correct header keys for each integration (#877)

Each integration handle header keys differently, this patch tries to
address these differences so that we have proper headers in responses.

**ALB Integration**
https://docs.aws.amazon.com/elasticloadbalancing/latest/application/lambda-functions.html#multi-value-headers-response

    The names of the fields used for headers differ depending on
    whether you enable multi-value headers for the target group.
    You must use multiValueHeaders if you have enabled multi-value
    headers and headers otherwise.

**APIGW v1 Integration**
https://docs.aws.amazon.com/apigateway/latest/developerguide/set-up-lambda-proxy-integrations.html#api-gateway-simple-proxy-for-lambda-output-format

    If you specify values for both headers and multiValueHeaders,
    API Gateway merges them into a single list. If the same key-value
    pair is specified in both, only the values from multiValueHeaders
    will appear in the merged list.

**APIGW v2 Integration**
https://docs.aws.amazon.com/apigateway/latest/developerguide/http-api-develop-integrations-lambda.html

    Format 2.0 doesn't have multiValueHeaders or
    multiValueQueryStringParameters fields. Duplicate headers are
    combined with commas and included in the headers field.
    Duplicate query strings are combined with commas and included
    in the queryStringParameters field.

**`awslabs/aws-lambda-go-api-proxy` source code**

- https://github.com/awslabs/aws-lambda-go-api-proxy/blob/3f6c8160ae0c22b0bd05b2e3a9122736f035c74b/core/response.go#L117
- https://github.com/awslabs/aws-lambda-go-api-proxy/blob/3f6c8160ae0c22b0bd05b2e3a9122736f035c74b/core/responseALB.go#L108
- https://github.com/awslabs/aws-lambda-go-api-proxy/blob/3f6c8160ae0c22b0bd05b2e3a9122736f035c74b/core/responsev2.go#L117

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 "multi_value_headers" field. Duplicate headers
+                    // are combined with commas and included in the headers field.
+                    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!(
@@ -121,8 +120,8 @@ 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,
             }),
@@ -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":[]}"#
         )
     }