fix casing issue when cloning Headers object

lib/web/fetch/headers.js

@@ -602,6 +602,12 @@ webidl.converters.HeadersInit = function (V) {
   if (webidl.util.Type(V) === 'Object') {
     const iterator = Reflect.get(V, Symbol.iterator)
 
+    // 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
+    }
+
     if (typeof iterator === 'function') {
       return webidl.converters['sequence<sequence<ByteString>>'](V, iterator.bind(V))
     }

lib/web/fetch/request.js

@@ -459,8 +459,9 @@ class Request {
       // 4. If headers is a Headers object, then for each header in its header
       // list, append header’s name/header’s value to this’s headers.
       if (headers instanceof HeadersList) {
-        for (const [key, val] of headers) {
-          headersList.append(key, val)
+        for (const { 0: key, 1: val } of headers) {
+          // Note: The header names are already in lowercase.
+          headersList.append(key, val, true)
         }
         // Note: Copy the `set-cookie` meta-data.
         headersList.cookies = headers.cookies

test/fetch/headers-case.js

@@ -0,0 +1,30 @@
+'use strict'
+
+const { fetch, Headers } = require('../..')
+const { createServer } = require('node:http')
+const { once } = require('node:events')
+const { test } = require('node:test')
+const { tspl } = require('@matteo.collina/tspl')
+
+test('Headers retain keys case-sensitive', async (t) => {
+  const assert = tspl(t, { plan: 3 })
+
+  const server = createServer((req, res) => {
+    assert.ok(req.rawHeaders.includes('Content-Type'))
+
+    res.end()
+  }).listen(0)
+
+  t.after(() => server.close())
+  await once(server, 'listening')
+
+  for (const headers of [
+    new Headers([['Content-Type', 'text/plain']]),
+    { 'Content-Type': 'text/plain' },
+    [['Content-Type', 'text/plain']]
+  ]) {
+    await fetch(`http://localhost:${server.address().port}`, {
+      headers
+    })
+  }
+})