Issue when setting maxAge = 'session'

Everything is fine when I set maxAge to a number.

As soon as I set `maxAge` to `"session"` I get a REDIS error:

```
ReplyError$1: ERR value is not an integer or out of range
```

I have launched my app with redis traces:
```
DEBUG=ioredis:* node app/server.js
```

And I see the error at this point:
```
ioredis:redis write command[35.224.153.24:18918]: 0 -> setex('ee336b93-7195-4e06-9e66-dffedbda5466,session,{"grant":{"provider":"google","dynamic":{"x":"151","y":"26"},"state":"98709c21509fd43b0c2ec39071816bebaf0f4368","nonce":"9de36d71dfba5d70310464eee0c4f3a486 ... ') +8s
```

This is how I setup the session:
```
app
.use(
session(
{
store: redisStore({
host: redisUrl.hostname,
port: Number(redisUrl.port),
password: redisUrl.password,
}),
maxAge: 'session',
key: 'flyxc.sess',
httpOnly: true,
signed: true,
rolling: false,
renew: false,
secure: process.env.NODE_ENV != 'development',
},
app,
),
)
```

Any clue what could go wrong or how to debug further ?

Reproduce the issue using the app/server.js setup and the session configuration shown in the report. Trace how maxAge: "session" reaches the Redis setex call, then verify the behavior with regression coverage; done means the configuration no longer produces an invalid Redis TTL.

Written by the indexing model from the issue text.