Improve performance of set method

<details> Hi, In our system, we store additional keys in Redis and need to sync certain data within session storage. To achieve this, we require a callback that triggers whenever a session is saved to Redis, allowing us to perform operations on our custom keys. Currently, as a workaround, we have overridden the set method of the Redis store: ```javascript const store = new RedisStore({ ... }); const originalSet = store.set; store.set = async function (...args) { const session = args?.[1]; // Custom logic return originalSet.apply(store, args); }; ``` However, this approach is fragile and requires validation each time a new version of connect-redis is released. To make this process more robust and maintainable, we propose adding a built-in onSet callback to `connect-redis`. This would provide a cleaner, more reliable way to execute custom logic when a session is saved. Let me know if you need refinements! Thanks in advance 🚀 </details> Above was the original request. Currently this PR aims to call the `serializer.stringify` when necessary.