fix: keep raw header name

lib/web/fetch/headers.js

@@ -250,7 +250,7 @@ class HeadersList {
   get entries () {
     const headers = {}
 
-    if (this[kHeadersMap].size) {
+    if (this[kHeadersMap].size !== 0) {
       for (const { name, value } of this[kHeadersMap].values()) {
         headers[name] = value
       }
@@ -259,6 +259,10 @@ class HeadersList {
     return headers
   }
 
+  rawValues () {
+    return this[kHeadersMap].values()
+  }
+
   get entriesList () {
     const headers = []
 

lib/web/fetch/request.js

@@ -477,9 +477,8 @@ 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 { 0: key, 1: val } of headers) {
-          // Note: The header names are already in lowercase.
-          headersList.append(key, val, true)
+        for (const { name, value } of headers.rawValues()) {
+          headersList.append(name, value, false)
         }
         // Note: Copy the `set-cookie` meta-data.
         headersList.cookies = headers.cookies

test/fetch/headers-case.js

@@ -1,30 +1,32 @@
 'use strict'
 
-const { fetch, Headers } = require('../..')
+const { fetch, Headers, Request } = require('../..')
 const { createServer } = require('node:http')
 const { once } = require('node:events')
 const { test } = require('node:test')
 const { tspl } = require('@matteo.collina/tspl')
+const { closeServerAsPromise } = require('../utils/node-http')
 
 test('Headers retain keys case-sensitive', async (t) => {
-  const assert = tspl(t, { plan: 3 })
+  const assert = tspl(t, { plan: 4 })
 
   const server = createServer((req, res) => {
     assert.ok(req.rawHeaders.includes('Content-Type'))
 
     res.end()
   }).listen(0)
 
-  t.after(() => server.close())
+  t.after(closeServerAsPromise(server))
   await once(server, 'listening')
 
+  const url = `http://localhost:${server.address().port}`
   for (const headers of [
     new Headers([['Content-Type', 'text/plain']]),
     { 'Content-Type': 'text/plain' },
     [['Content-Type', 'text/plain']]
   ]) {
-    await fetch(`http://localhost:${server.address().port}`, {
-      headers
-    })
+    await fetch(url, { headers })
   }
+  // see https://github.com/nodejs/undici/pull/3183
+  await fetch(new Request(url, { headers: [['Content-Type', 'text/plain']] }), { method: 'GET' })
 })