[9.x] Cast $perPage to integer on Paginator

[gdebrauwer]

The $perPage parameter of LengthAwarePaginator/CursorPaginator must be an integer according to docblocks, but when the parameter is an integer in a string, it is never cast to an integer. Such casting is already done on $lastPage and $currentPage, but not yet on $perPage. This PR fixes that.

[driesvints]

I don't think we should cast this. If you send in something else than an integer then there's a bug in your app somewhere and I think you should be made aware of that.

[gdebrauwer]

When you use the paginate-method with a "per page" value from a querystring parameter, then that value is an integer in a string. This is similar to the default page parameter in the querystring that is used by the paginator. The currentPage value is, because of that, also an integer as a string by default, but it is cast in the constructor of the LenghtAwarePaginator. So I think it is logical to do the same with the perPage value.

This ensures the json-encoded version of the paginator always returns per_page as an integer, because you now may get in the situation that per_page json value is sometimes a string and sometimes an integer

[gdebrauwer]

When you provided an actual string as perPage, not an integer in a string, you will still get an exception because the query will fail when using a string as a limit amount