fix(response): copy headers when init is a foreign Response

src/response.ts

@@ -46,21 +46,16 @@ export class Response {
   constructor(body?: BodyInit | null, init?: ResponseInit) {
     let headers: HeadersInit | undefined
     this.#body = body
-    if (init instanceof Response) {
+    if (init instanceof GlobalResponse) {
       const cachedGlobalResponse = (init as any)[responseCache]
       if (cachedGlobalResponse) {
         this.#init = cachedGlobalResponse
         // instantiate GlobalResponse cache and this object always returns value from global.Response
         this[getResponseCache]()
         return
-      } else {
-        this.#init = init.#init
-        // Read headers via the live getter so mutations made on `init` after
-        // construction (e.g. `init.headers.append('Set-Cookie', ...)`) are
-        // preserved on the clone. `new Headers(...)` still produces an
-        // independent copy so parent and child do not share the same object.
-        headers = new Headers(init.headers)
       }
+      this.#init = init instanceof Response ? init.#init : init
+      headers = new Headers(init.headers)
     } else {
       this.#init = init
     }

test/response.test.ts

@@ -139,6 +139,16 @@ describe('Response', () => {
     expect(res.headers.get('location')).toEqual('https://example.com/')
   })
 
+  it('Should copy headers when rebuilding a response from fetch()', async () => {
+    const upstream = await fetch(`http://localhost:${port}`)
+    const rebuilt = new Response(upstream.body, upstream)
+    rebuilt.headers.set('x-test', '1')
+    expect(rebuilt.headers.get('x-test')).toEqual('1')
+    expect(rebuilt.headers.get('content-type')).toEqual('application/json charset=UTF-8')
+    expect(upstream.headers.get('x-test')).toBeNull()
+    expect(await rebuilt.json()).toEqual({ status: 'ok' })
+  })
+
   it('Nested constructors should not cause an error even if ReadableStream is specified', async () => {
     const stream = new Response('hono').body
     const parentResponse = new Response(stream)