Pager object was keeping reference to old dataset in situations when
some additionap predicated were called after using #page or #per_page
methods, e.g.:

```
users.page(2).where(active: true)
```

In the example above, pager was still holding reference to a dataset not
having active=true condition set. This resulted in a correct dataset
being returned, but the pager methods like #total_pages returning wrong
results.

In this commit, a new public method #pager is created which shadows
regular usage from dry-initialize by firstly update the pager with fresh
dataset, so the calculations are correct.