Allow string[] as Record value on HeadersInit (nodejs#1244)

* fix(fetch): allow string[] as Record value on HeadersInit

* fix(fetch): allow readonly string arrays on HeadersInit

* refact(fetch): move HeadersInit test to test/fetch

test/fetch/headers.js

@@ -17,7 +17,7 @@ const {
 } = require('../../lib/fetch/constants')
 
 tap.test('Headers initialization', t => {
-  t.plan(6)
+  t.plan(7)
 
   t.test('allows undefined', t => {
     t.plan(1)
@@ -97,6 +97,20 @@ tap.test('Headers initialization', t => {
       ['key', 'value', 'value2']
     ]), 'throws when too many arguments are passed')
   })
+
+  t.test('accepts headers as objects with array values', t => {
+    t.plan(1)
+    const headers = new Headers({
+      a: ['1', '2'],
+      b: ['3', '4'],
+      c: '5'
+    })
+    t.same(headers.entries(), [
+      ['a', '1,2'],
+      ['b', '3,4'],
+      ['c', '5']
+    ])
+  })
 })
 
 tap.test('Headers append', t => {

types/fetch.d.ts

@@ -42,7 +42,7 @@ export interface BodyMixin {
   readonly text: () => Promise<string>
 }
 
-export type HeadersInit = string[][] | Record<string, string> | Headers
+export type HeadersInit = string[][] | Record<string, string | ReadonlyArray<string>> | Headers
 
 export declare class Headers implements Iterable<[string, string]> {
   constructor (init?: HeadersInit)
@@ -107,7 +107,7 @@ export interface RequestInit {
   readonly window?: null
 }
 
-export type ReferrerPolicy = 
+export type ReferrerPolicy =
   | ''
   | 'no-referrer'
   | 'no-referrer-when-downgrade'