refactor(network.fetch): handle iterable header form and tidy test

`Dispatcher.dispatch` accepts headers as a Map/web-Headers iterable in
addition to the flat string[] and plain object forms. The previous
object branch routed iterables through Object.entries, which would
silently drop every header for Map-like inputs. Detect Symbol.iterator
and consume the iterator directly when present.

Also drop the underscore prefix on the test's `req` parameter since it
is used.

Author: zkochan

network/fetch/src/dispatcher.ts

@@ -47,8 +47,14 @@ function stripSecFetchHeaders (dispatch: Dispatcher['dispatch']): Dispatcher['di
         }
         opts = { ...opts, headers: filtered }
       } else if (typeof opts.headers === 'object') {
+        // undici also accepts an iterable of [key, value] pairs (e.g. a Map or
+        // web Headers). Use that iterator when present; otherwise fall back to
+        // Object.entries for plain IncomingHttpHeaders objects.
+        const entries = Symbol.iterator in opts.headers
+          ? (opts.headers as Iterable<[string, string | string[] | undefined]>)
+          : Object.entries(opts.headers as Record<string, string | string[] | undefined>)
         const headers: Record<string, string | string[] | undefined> = {}
-        for (const [key, value] of Object.entries(opts.headers as Record<string, string | string[] | undefined>)) {
+        for (const [key, value] of entries) {
           if (!key.toLowerCase().startsWith('sec-fetch-')) {
             headers[key] = value
           }

network/fetch/test/fetchFromRegistry.test.ts

@@ -313,8 +313,8 @@ test('createDispatchedFetch returns a fetch bound to the given dispatcher option
 
 test('sec-fetch-* headers are stripped from requests', async () => {
   const receivedHeaders = await new Promise<http.IncomingHttpHeaders>((resolve, reject) => {
-    const server = http.createServer((_req, res) => {
-      resolve(_req.headers)
+    const server = http.createServer((req, res) => {
+      resolve(req.headers)
       res.writeHead(200, { 'content-type': 'application/json' })
       res.end('{"ok":true}')
     })