ExpiryMilliseconds-value is lost

Changing the ExpiryMilliseconds value affects the expiration of already saved values.

```
lscache.setExpiryMilliseconds(1)
undefined
lscache.set('t','asd',1);
undefined
lscache.get('t')
null
lscache.set('t','asd',1);
undefined
lscache.setExpiryMilliseconds(100)
undefined
lscache.get('t')
"asd"
```

The last `get()` should return `null`.