Fix zeroing of empty_bins in bin()
Fix zeroing of empty_bins in bin()

Something like

```
p = ggplot(aes(x="price"), data=diamonds) + geom_histogram()
p = p + facet_wrap("cut")
p + xlim(0, 10000)
```

was failing. The count of the first bin was being zeroed instead
of inserting a zero for the zeroth bin.

has2k1/plotnine,has2k1/plotnine