MongoExpiringSession should calculate expireAt when getExpireAt is accessed

Using spring-session with MongoDB

MongoExpiringSession's expireAt field is set in the the implementation of AbstractMongoSessionConverter's method
MongoExpiringSession convert(DBObject sessionWrapper)
and when MongoExpiringSession's method setLastAccessedTime is invoked.

In some circumstances when I change MongoExpiringSession's interval field through
request.getSession().setMaxInactiveInterval(...)
interval field is updated in MongoDB, but the expireAt field is not updated

I think that this can be solved by calculating expireAt field when getExpireAt is accessed.
In fact I think that MongoExpiringSession does not need this field and getExpireAt method should be something like

new Date(getLastAccessedTime() + TimeUnit.SECONDS.toMillis(this.interval));

Start with MongoExpiringSession.getExpireAt(), setLastAccessedTime(), and AbstractMongoSessionConverter.convert(DBObject sessionWrapper), which the issue identifies as the relevant entry points. Reproduce changing the session max inactive interval and verify that the persisted expireAt value reflects the updated interval when getExpireAt() is accessed.