Fixed error “Cannot read property ‘slice’ of undefined”

Error text:
```
TypeError: Cannot read property 'slice' of undefined
    at searchEntireRow (http://0.0.0.0:9000/vendor/ng-grid/ng-grid.debug.js:2613:57)
    at filterFunc (http://0.0.0.0:9000/vendor/ng-grid/ng-grid.debug.js:2658:26)
    at http://0.0.0.0:9000/vendor/ng-grid/ng-grid.debug.js:2674:24
    at Array.filter (native)
    at self.evalFilter (http://0.0.0.0:9000/vendor/ng-grid/ng-grid.debug.js:2673:47)
    at Object.fn (http://0.0.0.0:9000/vendor/ng-grid/ng-grid.debug.js:2780:18)
    at Scope.$digest (http://0.0.0.0:9000/bower_components/angular/angular.js:11789:29)
    at Scope.$apply (http://0.0.0.0:9000/bower_components/angular/angular.js:12042:24)
    at HTMLInputElement.listener (http://0.0.0.0:9000/bower_components/angular/angular.js:16040:15)
    at HTMLInputElement.jQuery.event.dispatch (http://0.0.0.0:9000/bower_components/jquery/jquery.js:5095:9) 
```
Error occurs when you define a `cellFilter` for a field, using a filter with no parameter like `cellFilter: "lowercase"`.
I have added a unit test to prove this and a plunker to show the error: http://plnkr.co/edit/wso8bo