Bug 28074: Make current_search.offset an integer instead of string

Bug 28074: Make current_search.offset an integer instead of string

To test:
1. Do a search with 3-4 results, go to the record and use the browse
   controls, when you get to the last record if you attempt to go to the
   'next' you'll end up on a page which says 'record not found.'
2. Do a search with exactly 10 results. Click on the first record, the
   'next' arrow is not a link, you cannot move to the next result.
3. Apply patch
4. Repeat steps 1-2 and the problems should be gone.

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>