Updated Options -> message type

Updated Options -> message type to mirror express typings, ran into this issue when trying to use an object as the message value.

types/express-rate-limit/index.d.ts

@@ -22,7 +22,7 @@ declare namespace RateLimit {
         headers?: boolean;
         keyGenerator?(req: express.Request, res: express.Response): string;
         max?: number;
-        message?: string;
+        message?: string | Buffer | Object;
         skip?(req: express.Request, res: express.Response): boolean;
         skipFailedRequests?: boolean;
         statusCode?: number;