feat(events): add MultiValueHeaders to LambdaFunctionURLResponse

AWS Lambda Function URLs can return multiple values for the same header
key (e.g. multiple Set-Cookie headers). Without MultiValueHeaders, callers
had to work around the single-value Headers map and could not set more than
one cookie in a single response.

Adds MultiValueHeaders map[string][]string with omitempty, matching the
shape already present on APIGatewayProxyResponse and
APIGatewayV2HTTPResponse, and adds a round-trip marshaling test with a
multi-cookie fixture.

Fixes #556

Author: Nghĩa Nguyễn Ngọc

events/lambda_function_urls.go

@@ -61,11 +61,12 @@ type LambdaFunctionURLRequestContextHTTPDescription struct {
 
 // LambdaFunctionURLResponse configures the HTTP response to be returned by Lambda Function URL for the request.
 type LambdaFunctionURLResponse struct {
-	StatusCode      int               `json:"statusCode"`
-	Headers         map[string]string `json:"headers"`
-	Body            string            `json:"body"`
-	IsBase64Encoded bool              `json:"isBase64Encoded"`
-	Cookies         []string          `json:"cookies"`
+	StatusCode        int                 `json:"statusCode"`
+	Headers           map[string]string   `json:"headers"`
+	MultiValueHeaders map[string][]string `json:"multiValueHeaders,omitempty"`
+	Body              string              `json:"body"`
+	IsBase64Encoded   bool                `json:"isBase64Encoded"`
+	Cookies           []string            `json:"cookies"`
 }
 
 // LambdaFunctionURLStreamingResponse models the response to a Lambda Function URL when InvokeMode is RESPONSE_STREAM.

events/lambda_function_urls_test.go

@@ -60,6 +60,27 @@ func TestLambdaFunctionURLRequestMarshaling(t *testing.T) {
 	assert.JSONEq(t, string(inputJSON), string(outputJSON))
 }
 
+func TestLambdaFunctionURLResponseMultiValueHeadersMarshaling(t *testing.T) {
+	inputJSON, err := ioutil.ReadFile("./testdata/lambda-urls-response-multi-value-headers.json")
+	if err != nil {
+		t.Errorf("could not open test file. details: %v", err)
+	}
+
+	var inputEvent LambdaFunctionURLResponse
+	if err := json.Unmarshal(inputJSON, &inputEvent); err != nil {
+		t.Errorf("could not unmarshal event. details: %v", err)
+	}
+
+	require.Equal(t, []string{"a=1; Path=/", "b=2; Path=/"}, inputEvent.MultiValueHeaders["Set-Cookie"])
+
+	outputJSON, err := json.Marshal(inputEvent)
+	if err != nil {
+		t.Errorf("could not marshal event. details: %v", err)
+	}
+
+	assert.JSONEq(t, string(inputJSON), string(outputJSON))
+}
+
 func TestLambdaFunctionURLStreamingResponseMarshaling(t *testing.T) {
 	for _, test := range []struct {
 		name         string

events/testdata/lambda-urls-response-multi-value-headers.json

@@ -0,0 +1,12 @@
+{
+    "statusCode": 200,
+    "headers": {
+        "Content-Type": "application/json"
+    },
+    "multiValueHeaders": {
+        "Set-Cookie": ["a=1; Path=/", "b=2; Path=/"]
+    },
+    "body": "",
+    "cookies": [],
+    "isBase64Encoded": false
+}