Fix apollo-server-env mock

__mocks__/apollo-server-env.ts

@@ -46,16 +46,16 @@ mockFetch.mockJSONResponseOnce = (
   );
 };
 
-export {
+const env = {
-  mockFetch as fetch,
+  fetch: mockFetch,
   Request,
-  RequestInit,
   Response,
   Body,
-  BodyInit,
   Headers,
-  HeadersInit,
   URL,
   URLSearchParams,
-  URLSearchParamsInit,
 };
+
+jest.doMock('apollo-server-env', () => env);
+
+export = env;

packages/apollo-datasource-rest/src/HTTPCache.ts

@@ -160,7 +160,7 @@ function policyResponseFrom(response: Response) {
 
 function headersToObject(headers: Headers) {
   const object = Object.create(null);
-  for (const [name, value] of headers as Headers) {
+  for (const [name, value] of headers) {
     object[name] = value;
   }
   return object;