Fix onTableLength assigning currentPerPage as string

[SafAlhaji]

When changing the number of rows per page, the currentPerPage property gets assigned to a string, and not a number. This results in not viewing all rows when choosing "All", as the identity (triple equality, w/e) operator in the following line will return false:

paginatedRows = paginatedRows.slice((this.currentPage - 1) * this.currentPerPage, this.currentPerPage === -1 ? paginatedRows.length + 1 : this.currentPage * this.currentPerPage);

[thepill]

thanks @SafAlhaji