Fixed 'last row overlapped by scrollbar' bug

On a grid without horizontal scrollbar, a resize of canvas that make scrollbar appear will make the last row overlapped by the scrollbar, inaccessible.

This bug appeared at stackoverflow on the question ["slickgrid resize causes scrollbar gap near bottom"](http://stackoverflow.com/questions/13178547/slickgrid-resize-causes-scrollbar-gap-near-bottom), and I found the answer of the user [Aman Mahajan](http://stackoverflow.com/users/2048375/aman-mahajan) as the solution, I just applied the proposed changes here, and the result works like a charm!