fix(ext/node): don't concat set-cookie in ServerResponse.appendHeader (#25000)

Follow-on to
https://github.com/denoland/deno/pull/24216/files#r1642188672

ext/node/polyfills/http.ts

@@ -1439,12 +1439,11 @@ export class ServerResponse extends NodeWritable {
   }
 
   appendHeader(name: string, value: string | string[]) {
-    if (Array.isArray(value)) {
-      this.#hasNonStringHeaders = true;
-    }
     if (this.#headers[name] === undefined) {
+      if (Array.isArray(value)) this.#hasNonStringHeaders = true;
       this.#headers[name] = value;
     } else {
+      this.#hasNonStringHeaders = true;
       if (!Array.isArray(this.#headers[name])) {
         this.#headers[name] = [this.#headers[name]];
       }

tests/unit_node/http_test.ts

@@ -1118,6 +1118,27 @@ Deno.test("[node/http] ServerResponse appendHeader", async () => {
   await promise;
 });
 
+Deno.test("[node/http] ServerResponse appendHeader set-cookie", async () => {
+  const { promise, resolve } = Promise.withResolvers<void>();
+  const server = http.createServer((_req, res) => {
+    res.appendHeader("Set-Cookie", "a=b");
+    res.appendHeader("Set-Cookie", "c=d");
+    res.end("Hello World");
+  });
+
+  server.listen(async () => {
+    const { port } = server.address() as { port: number };
+    const res = await fetch(`http://localhost:${port}`);
+    assertEquals(res.headers.getSetCookie(), ["a=b", "c=d"]);
+    assertEquals(await res.text(), "Hello World");
+    server.close(() => {
+      resolve();
+    });
+  });
+
+  await promise;
+});
+
 Deno.test("[node/http] IncomingMessage override", () => {
   const req = new http.IncomingMessage(new net.Socket());
   // https://github.com/dougmoscrop/serverless-http/blob/3aaa6d0fe241109a8752efb011c242d249f32368/lib/request.js#L20-L30