removing the count(*) when no column assigned or :all #240

Trying again from a different branch I was getting errors and I started over.
Originally this one https://github.com/amatsuda/kaminari/pull/462
Hi @zzak this is a silly change but it will make a huge difference to not do an extra select count(*) when is not required. This change can help us to solve this issue gregbell/active_admin#2638 in Active Admin gem cc @seanlinsley