fix(extollo/extollo#4): explicitly define return type to fix build error

src/http/lifecycle/Response.ts

@@ -1,6 +1,7 @@
 import {Request} from "./Request";
 import {ErrorWithContext, HTTPStatus, BehaviorSubject} from "@extollo/util"
 import {ServerResponse} from "http"
+import {HTTPCookieJar} from "../kernel/HTTPCookieJar";
 
 /**
  * Error thrown when the server tries to re-send headers after they have been sent once.
@@ -78,7 +79,7 @@ export class Response {
     }
 
     /** Get the HTTPCookieJar for the client. */
-    public get cookies() {
+    public get cookies(): HTTPCookieJar {
         return this.request.cookies
     }