add test

packages/kit/test/prerendering/basics/src/routes/immutable-headers/+server.js

@@ -0,0 +1,8 @@
+export const prerender = true;
+
+export const GET = () => {
+	const response = new Response('foo');
+	// this simulates immutable Response Headers, like those returned by undici
+	Object.defineProperty(response.headers, 'set', { value: null });
+	return response;
+};

packages/kit/test/prerendering/basics/test/tests.spec.js

@@ -223,3 +223,8 @@ test('prerendered.paths omits trailing slashes for endpoints', () => {
 		expect(content, `Missing ${path}`).toMatch(`"${path}"`);
 	}
 });
+
+test('prerenders responses with immutable Headers', () => {
+	const content = read('immutable-headers');
+	expect(content).toMatch('foo');
+});