Filtering problem when grouped

[cschwaderer]

I've found a problem for filtering/searching multi-level grouped input models, so, I changed the if condition in line 182/183

 if ( typeof $scope.filteredModel[ $scope.filteredModel.length - 1 ][ attrs.groupProperty ] !== 'undefined'
                            && $scope.filteredModel[ $scope.filteredModel.length - 1 ][ attrs.groupProperty ] === false )

into

if (
                           (
                               !$scope.filteredModel[ $scope.filteredModel.length - 1 ]
                               ||
                               typeof $scope.filteredModel[ $scope.filteredModel.length - 1 ][ attrs.groupProperty ] !== 'undefined'
                           )
                           &&
                           (
                               !$scope.filteredModel[ $scope.filteredModel.length - 1 ]
                               ||
                               $scope.filteredModel[ $scope.filteredModel.length - 1 ][ attrs.groupProperty ] === false
                           )

That solved the problem for me.
I don't see a point in creating a PR, since it seems they are not eagerly pulled right now anyway. However, feel free the use the fix wherever you want!