change default header to node (#2310)

* change default header to `node`

* switch on filename, add tests

lib/fetch/index.js

@@ -1344,7 +1344,7 @@ async function httpNetworkOrCacheFetch (
   //    user agents should append `User-Agent`/default `User-Agent` value to
   //    httpRequest’s header list.
   if (!httpRequest.headersList.contains('user-agent')) {
-    httpRequest.headersList.append('user-agent', 'undici')
+    httpRequest.headersList.append('user-agent', __filename.endsWith('index.js') ? 'undici' : 'node')
   }
 
   //    15. If httpRequest’s cache mode is "default" and httpRequest’s header

test/fetch/user-agent.js

@@ -0,0 +1,25 @@
+'use strict'
+
+const { test } = require('tap')
+const events = require('events')
+const http = require('http')
+const undici = require('../../')
+const nodeBuild = require('../../undici-fetch.js')
+
+test('user-agent defaults correctly', async (t) => {
+  const server = http.createServer((req, res) => {
+    res.end(JSON.stringify({ userAgentHeader: req.headers['user-agent'] }))
+  })
+  t.teardown(server.close.bind(server))
+
+  server.listen(0)
+  await events.once(server, 'listening')
+  const url = `http://localhost:${server.address().port}`
+  const [nodeBuildJSON, undiciJSON] = await Promise.all([
+    nodeBuild.fetch(url).then((body) => body.json()),
+    undici.fetch(url).then((body) => body.json())
+  ])
+
+  t.same(nodeBuildJSON.userAgentHeader, 'node')
+  t.same(undiciJSON.userAgentHeader, 'undici')
+})