Start double picker as blank range

[windmaomao]

I wasn't able to use double picker starting with a blank range, the problem is that if my data is { from: '', to: '' }, the picker will not let me select anything.

So I dug into the library and found if I can make the following change, it'll work. Basically, I just check is the model is empty.

          _withinLimits = function(day, month) {
            var withinLimits;
            withinLimits = true;
            if ($scope.model && $scope.model.isValid()) {
              if ($scope.before) {
                withinLimits && (withinLimits = day.isSameOrBefore($scope.before, 'day'));
              }
              if ($scope.after) {
                withinLimits && (withinLimits = day.isSameOrAfter($scope.after, 'day'));
              }
            }
            return withinLimits;
          };