add sessionExpire option

example/test.ts

@@ -1,6 +1,6 @@
 import { Status } from "https://deno.land/std@0.186.0/http/http_status.ts";
 import prettyTime from "npm:pretty-time";
-import { HTTPServer } from "../mod.ts";
+import { HTTPServer, SessionExpire } from "../mod.ts";
 
 const JOKES = [
   "Why do Java developers often wear glasses? They can't C#.",
@@ -194,4 +194,5 @@ httpServer.listen({
   staticLocalDir: "/static",
   staticServePath: "/assets",
   sessionSecret: "SuperDuperSecret",
+  sessionExpire: SessionExpire.NEVER
 });