Update exposer.js

Correct situation when the consumer invokeAction("sayOk") returns "undefined".

Author: IvoVargas

examples/security/oauth/exposer.js

@@ -29,6 +29,7 @@ const td = {
         sayOk: {
             description: "A simple action protected with oauth",
             idempotent: true,
+            output: { type: "string" },
         },
     },
 };