fix header cloning bug

lib/web/fetch/headers.js

@@ -259,6 +259,24 @@ class HeadersList {
     return headers
   }
 
+  get entriesList () {
+    const headers = []
+
+    if (this[kHeadersMap].size !== 0) {
+      for (const { 0: lowerName, 1: { name, value } } of this[kHeadersMap]) {
+        if (lowerName === 'set-cookie') {
+          for (const cookie of this.cookies) {
+            headers.push(['set-cookie', cookie])
+          }
+        } else {
+          headers.push([name, value])
+        }
+      }
+    }
+
+    return headers
+  }
+
   // https://fetch.spec.whatwg.org/#convert-header-names-to-a-sorted-lowercase-set
   toSortedArray () {
     const size = this[kHeadersMap].size
@@ -605,7 +623,7 @@ webidl.converters.HeadersInit = function (V) {
     // A work-around to ensure we send the properly-cased Headers when V is a Headers object.
     // Read https://github.com/nodejs/undici/pull/3159#issuecomment-2075537226 before touching, please.
     if (!util.types.isProxy(V) && kHeadersList in V && iterator === Headers.prototype.entries) { // Headers object
-      return V[kHeadersList].entries
+      return V[kHeadersList].entriesList
     }
 
     if (typeof iterator === 'function') {

test/fetch/headers.js

@@ -725,6 +725,12 @@ test('Headers.prototype.getSetCookie', async (t) => {
     assert.deepStrictEqual(res.headers.getSetCookie(), ['test=onetwo', 'test=onetwothree'])
     assert.ok('set-cookie' in entries)
   })
+
+  await t.test('When Headers are cloned, so are the cookies (Headers constructor)', () => {
+    const headers = new Headers([['set-cookie', 'a'], ['set-cookie', 'b']])
+
+    assert.deepStrictEqual([...headers], [...new Headers(headers)])
+  })
 })
 
 test('When the value is updated, update the cache', (t) => {