Pager $watchers will not fire if sum of totalItems and page size stays the same

As mentioned by @c0bra at https://github.com/brianchance/ng-grid/commit/45e1b5c58a621b36f4fdd9cfed85544d5bb2fa36#commitcomment-8565148 

if decrement and increment of the totalItems and page size don't change the overall sum within one digest cycle then watchers will not fire. Changing the $watch expression to a function which compares string values fixes this.