add missing Headers interface augmentation from lib.dom.iterable.d.ts

Added test.ts to support type checking the type declaration files.

declaration files were generated and tested with the following command:

npm i --no-save typescript@4.2.3 \
&& cat <<EOF > .ts-graftrc.yaml \
&& npx ts-graft@1.0.1 \
&& rm .ts-graftrc.yaml \
&& npx tsc --lib ES2016 --target ES2016 --noEmit test.ts \
&& npm un typescript
grafts:
- source: typescript/lib/lib.dom.d.ts
  output: lib.fetch.d.ts
  include:
    - BodyInit
    - HeadersInit
    - RequestInfo
    - RequestInit
    - Response
    - ResponseInit
- source: typescript/lib/lib.dom.iterable.d.ts
  output: lib.fetch.dom.iterable.d.ts
  include:
    - Headers
EOF

index.d.ts

@@ -1,13 +1,15 @@
-import {
+import type {
   BodyInit,
-  Headers,
+  Headers as BaseHeaders,
   HeadersInit,
   Request,
   RequestInfo,
   RequestInit,
   Response,
   ResponseInit,
 } from "./lib.fetch"
+import type { Headers as IterHeaders } from "./lib.fetch.dom.iterable";
+type Headers = BaseHeaders & IterHeaders;
 
 export const fetch: (input: RequestInfo, init?: RequestInit) => Promise<Response>;
 

lib.fetch.dom.iterable.d.ts

@@ -0,0 +1,16 @@
+// Generated by resolving typescript/lib/lib.dom.iterable.d.ts from typescript@4.2.3
+export interface Headers {
+    [Symbol.iterator](): IterableIterator<[string, string]>;
+    /**
+     * Returns an iterator allowing to go through all key/value pairs contained in this object.
+     */
+    entries(): IterableIterator<[string, string]>;
+    /**
+     * Returns an iterator allowing to go through all keys of the key/value pairs contained in this object.
+     */
+    keys(): IterableIterator<string>;
+    /**
+     * Returns an iterator allowing to go through all values of the key/value pairs contained in this object.
+     */
+    values(): IterableIterator<string>;
+}

test.ts

@@ -0,0 +1,6 @@
+import { Headers } from "./index";
+
+const headers = new Headers();
+for (const h of headers) {
+
+}