node-fetch: correct Headers.raw type (#27364)

* `node-fetch`: add test for `Headers.raw`

* `node-fetch`: `Headers.raw`: add `string[]` to index signature value type

* `node-fetch`: `Headers.raw`: add `undefined` to index signature value type

* `node-fetch`: enable `strictNullChecks` and fix errors

* Revert "`node-fetch`: `Headers.raw`: add `undefined` to index signature value type"

This reverts commit 15ad14d72c42fa1f3ba472bc15f38a88230f3a4f.

* Fix expect type format

* Remove string

types/node-fetch/index.d.ts

@@ -102,7 +102,7 @@ export class Headers implements Iterable<[string, string]> {
     get(name: string): string | null;
     getAll(name: string): string[];
     has(name: string): boolean;
-    raw(): { [k: string]: string };
+    raw(): { [k: string]: string[] };
     set(name: string, value: string): void;
 
     // Iterator methods

types/node-fetch/node-fetch-tests.ts

@@ -44,7 +44,7 @@ function test_fetchUrlWithRequestObject() {
 	const request: Request = new Request("http://www.andlabs.net/html5/uCOR.php", requestOptions);
 	const timeout: number = request.timeout;
 	const size: number = request.size;
-	const agent: Agent = request.agent;
+	const agent: Agent | undefined = request.agent;
 	const protocol: string = request.protocol;
 
 	handlePromise(fetch(request));
@@ -71,3 +71,9 @@ function handlePromise(promise: Promise<Response>, isArrayBuffer: boolean = fals
 		console.log(text);
 	});
 }
+
+function test_headersRaw() {
+	const headers = new Headers();
+	const myHeader = 'foo';
+	headers.raw()[myHeader]; // $ExpectType string[]
+}

types/node-fetch/tsconfig.json

@@ -7,7 +7,7 @@
         ],
         "noImplicitAny": true,
         "noImplicitThis": true,
-        "strictNullChecks": false,
+        "strictNullChecks": true,
         "strictFunctionTypes": true,
         "baseUrl": "../",
         "typeRoots": [