Revert "fix casing issue when cloning Headers object (#3160)"

This reverts commit 034e3f0.

lib/web/fetch/headers.js

@@ -602,12 +602,6 @@ 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

@@ -459,9 +459,8 @@ 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 [key, val] of headers) {
+          headersList.append(key, val)
         }
         // Note: Copy the `set-cookie` meta-data.
         headersList.cookies = headers.cookies