fix(ext/node): add ServerResponse#appendHeader (denoland#24216)

ext/node/polyfills/http.ts

@@ -1420,6 +1420,26 @@ export class ServerResponse extends NodeWritable {
     return this;
   }
 
+  appendHeader(name: string, value: string | string[]) {
+    if (Array.isArray(value)) {
+      this.#hasNonStringHeaders = true;
+    }
+    if (this.#headers[name] === undefined) {
+      this.#headers[name] = value;
+    } else {
+      if (!Array.isArray(this.#headers[name])) {
+        this.#headers[name] = [this.#headers[name]];
+      }
+      const header = this.#headers[name];
+      if (Array.isArray(value)) {
+        header.push(...value);
+      } else {
+        header.push(value);
+      }
+    }
+    return this;
+  }
+
   getHeader(name: string) {
     return this.#headers[name];
   }

tests/unit_node/http_test.ts

@@ -968,6 +968,32 @@ Deno.test("[node/http] ServerResponse getHeader", async () => {
   await promise;
 });
 
+Deno.test("[node/http] ServerResponse appendHeader", async () => {
+  const { promise, resolve } = Promise.withResolvers<void>();
+  const server = http.createServer((_req, res) => {
+    res.setHeader("foo", "bar");
+    res.appendHeader("foo", "baz");
+    res.appendHeader("foo", ["qux"]);
+    res.appendHeader("foo", ["quux"]);
+    res.appendHeader("Set-Cookie", "a=b");
+    res.appendHeader("Set-Cookie", ["c=d", "e=f"]);
+    res.end("Hello World");
+  });
+
+  server.listen(async () => {
+    const { port } = server.address() as { port: number };
+    const res = await fetch(`http://localhost:${port}`);
+    assertEquals(res.headers.get("foo"), "bar, baz, qux, quux");
+    assertEquals(res.headers.getSetCookie(), ["a=b", "c=d", "e=f"]);
+    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