fix: prevent crash when using custom headers

It seems that the Deno.upgradeWebSocket() method does not allow to
provide additional headers during the WebSocket handshake.

Exception:

> TypeError: Headers are immutable.
>    at Headers.set (deno:ext/fetch/20_headers.js:384:15)
>    at /packages/engine.io/lib/transports/websocket.ts:60:24
>    at Headers.forEach (deno:ext/webidl/00_webidl.js:1001:13)
>    at WS.onRequest (/packages/engine.io/lib/transports/websocket.ts:59:21)
>    at Server.handshake (/packages/engine.io/lib/server.ts:344:31)

packages/engine.io/lib/server.ts

@@ -214,7 +214,7 @@ export class Server extends EventEmitter<never, never, ServerReservedEvents> {
       if (req.headers.has("upgrade")) {
         const transport = new WS(this.opts);
 
-        const promise = transport.onRequest(req, responseHeaders);
+        const promise = transport.onRequest(req);
 
         socket._maybeUpgrade(transport);
 

packages/engine.io/lib/transports/websocket.ts

@@ -23,7 +23,7 @@ export class WS extends Transport {
     }
   }
 
-  public onRequest(req: Request, responseHeaders: Headers): Promise<Response> {
+  public onRequest(req: Request): Promise<Response> {
     const { socket, response } = Deno.upgradeWebSocket(req);
 
     this.socket = socket;
@@ -56,9 +56,7 @@ export class WS extends Transport {
       this.onClose();
     };
 
-    responseHeaders.forEach((value, key) => {
-      response.headers.set(key, value);
-    });
+    // note: response.headers is immutable, so it seems we can't add headers here
 
     return Promise.resolve(response);
   }

packages/engine.io/test/response_headers.test.ts

@@ -1,6 +1,11 @@
 import { assertEquals, describe, it } from "../../../test_deps.ts";
 import { Server } from "../lib/server.ts";
-import { enableLogs, parseSessionID, testServe } from "./util.ts";
+import {
+  enableLogs,
+  parseSessionID,
+  testServe,
+  testServeWithAsyncResults,
+} from "./util.ts";
 
 await enableLogs();
 
@@ -43,4 +48,23 @@ describe("response headers", () => {
       await dataResponse.body?.cancel();
     });
   });
+
+  it("should not crash when using WebSocket (noop)", () => {
+    const engine = new Server({
+      editHandshakeHeaders: (responseHeaders) => {
+        responseHeaders.set("abc", "123");
+      },
+      editResponseHeaders: (responseHeaders) => {
+        responseHeaders.set("def", "456");
+      },
+    });
+
+    return testServeWithAsyncResults(engine, 1, (port, done) => {
+      const socket = new WebSocket(
+        `ws://localhost:${port}/engine.io/?EIO=4&transport=websocket`,
+      );
+
+      socket.onopen = done;
+    });
+  });
 });