fix(Request): allow passing Headers instance to init.headers

lib/fetch/request.js

@@ -420,10 +420,9 @@ 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 Headers) {
-        // TODO (fix): Why doesn't this work?
-        // for (const [key, val] of headers[kHeadersList]) {
-        //   this[kHeaders].append(key, val)
-        // }
+        for (const [key, val] of headers) {
+          this[kHeaders].append(key, val)
+        }
 
         this[kState].headersList = new HeadersList([
           ...this[kState].headersList,

test/fetch/request.js

@@ -4,7 +4,8 @@
 
 const { test } = require('tap')
 const {
-  Request
+  Request,
+  Headers
 } = require('../../')
 const { kState } = require('../../lib/fetch/symbols.js')
 
@@ -272,3 +273,35 @@ test('post aborted signal cloned', t => {
   })
   ac.abort()
 })
+
+test('Passing headers in init', (t) => {
+  // https://github.com/nodejs/undici/issues/1400
+  t.test('Headers instance', (t) => {
+    const req = new Request('http://localhost', {
+      headers: new Headers({ key: 'value' })
+    })
+
+    t.equal(req.headers.get('key'), 'value')
+    t.end()
+  })
+
+  t.test('key:value object', (t) => {
+    const req = new Request('http://localhost', {
+      headers: { key: 'value' }
+    })
+
+    t.equal(req.headers.get('key'), 'value')
+    t.end()
+  })
+
+  t.test('[key, value][]', (t) => {
+    const req = new Request('http://localhost', {
+      headers: [['key', 'value']]
+    })
+
+    t.equal(req.headers.get('key'), 'value')
+    t.end()
+  })
+
+  t.end()
+})