geom_hex/stat_hex binwidth argument no longer working

ggplot2.1.0

``` r
> ggplot(mtcars) + geom_hex(aes(mpg, mpg), bins=5)
# works fine
```

``` r
> ggplot(mtcars) + geom_hex(aes(mpg, mpg), binwidth=2)
Warning message:
Computation failed in `stat_binhex()`:
missing value where TRUE/FALSE needed 
```

May be related to: https://github.com/hadley/ggplot2/issues/1608
