perf(headers): a single set-cookie

benchmarks/fetch/headers-length32.mjs

@@ -19,7 +19,7 @@ const headers = new Headers(
     'Width',
     'Accept-CH',
     'Via',
-    'Refresh',
+    'Set-Cookie',
     'Server',
     'Sec-Fetch-Dest',
     'Sec-CH-UA-Model',

lib/web/fetch/headers.js

@@ -536,7 +536,7 @@ class Headers {
     const cookies = this[kHeadersList].cookies
 
     // fast-path
-    if (cookies === null) {
+    if (cookies === null || cookies.length === 1) {
       // Note: The non-null assertion of value has already been done by `HeadersList#toSortedArray`
       return (this[kHeadersList][kHeadersSortedMap] = names)
     }

test/fetch/headers.js

@@ -694,7 +694,7 @@ test('Headers.prototype.getSetCookie', async (t) => {
   })
 
   // https://github.com/nodejs/undici/issues/1935
-  await t.test('When Headers are cloned, so are the cookies', async (t) => {
+  await t.test('When Headers are cloned, so are the cookies (single entry)', async (t) => {
     const server = createServer((req, res) => {
       res.setHeader('Set-Cookie', 'test=onetwo')
       res.end('Hello World!')
@@ -709,6 +709,22 @@ test('Headers.prototype.getSetCookie', async (t) => {
     assert.deepStrictEqual(res.headers.getSetCookie(), ['test=onetwo'])
     assert.ok('set-cookie' in entries)
   })
+
+  await t.test('When Headers are cloned, so are the cookies (multiple entries)', async (t) => {
+    const server = createServer((req, res) => {
+      res.setHeader('Set-Cookie', ['test=onetwo', 'test=onetwothree'])
+      res.end('Hello World!')
+    }).listen(0)
+
+    await once(server, 'listening')
+    t.after(closeServerAsPromise(server))
+
+    const res = await fetch(`http://localhost:${server.address().port}`)
+    const entries = Object.fromEntries(res.headers.entries())
+
+    assert.deepStrictEqual(res.headers.getSetCookie(), ['test=onetwo', 'test=onetwothree'])
+    assert.ok('set-cookie' in entries)
+  })
 })
 
 test('When the value is updated, update the cache', (t) => {