Fix ArgumentError when plotting Geom.bar with empty array of values

[JohnHBrock]

The below code was resulting in ArgumentError: reducing over an empty collection is not allowed because there is no value where category1 is B and category2 is Y.

using Gadfly

Gadfly.plot(x=[1, 4, 2],
            y=[5, 7, 3],
            xgroup=["A", "A", "B"],
            ygroup=["X", "Y", "Y"],
            Geom.subplot_grid(Geom.bar))

I fixed this by copying the logic used in the HistogramStatistic version of apply_statistic and added a test.

[coveralls]

Coverage increased (+2.4%) to 65.442% when pulling 9960d63 on JohnHBrock:pull-request/9960d63f into 91fb468 on dcjones:master.

[tlnagy]

Thanks for this fix, I'm rebooting travis and I'll merge this once tests pass. Sorry this took so long!