fastify: Switch req type to ServerResponse rather than `OutgoingM…

…essage`.

While currently we use `OutgoingMessage`, it actually seems to be suggested
that `ServerResonse` — which is an extension of `OutgoingMessage`:

https://github.com/fastify/fastify/blob/master/docs/TypeScript.md#example

Helps-to-land: #2844

packages/apollo-server-fastify/src/ApolloServer.ts

@@ -8,7 +8,7 @@ import {
   processFileUploads,
 } from 'apollo-server-core';
 import { FastifyInstance, FastifyReply, FastifyRequest } from 'fastify';
-import { IncomingMessage, OutgoingMessage, Server } from 'http';
+import { IncomingMessage, ServerResponse, Server } from 'http';
 import { graphqlFastify } from './fastifyApollo';
 import { GraphQLOperation } from 'graphql-upload';
 
@@ -36,7 +36,7 @@ const fileUploadMiddleware = (
   server: ApolloServerBase,
 ) => (
   req: FastifyRequest<IncomingMessage>,
-  reply: FastifyReply<OutgoingMessage>,
+  reply: FastifyReply<ServerResponse>,
   done: (err: Error | null, body?: any) => void,
 ) => {
   if (
@@ -80,7 +80,7 @@ export class ApolloServer extends ApolloServerBase {
     const promiseWillStart = this.willStart();
 
     return async (
-      app: FastifyInstance<Server, IncomingMessage, OutgoingMessage>,
+      app: FastifyInstance<Server, IncomingMessage, ServerResponse>,
     ) => {
       await promiseWillStart;
 
@@ -121,7 +121,7 @@ export class ApolloServer extends ApolloServerBase {
           const beforeHandlers = [
             (
               req: FastifyRequest<IncomingMessage>,
-              reply: FastifyReply<OutgoingMessage>,
+              reply: FastifyReply<ServerResponse>,
               done: () => void,
             ) => {
               // Note: if you enable playground in production and expect to be able to see your