nested (list) columns don't get a color in vis_dat(palette = "qual")

Using `vis_dat(palette = "qual")` on a tibble with a nested (list) column does not give a color to the list column, making them appear to be all NAs.

``` r
require(tidyverse)
#> Loading required package: tidyverse
require(visdat)
#> Loading required package: visdat

mtcars %>% 
  nest(gearcarb = c(gear, carb)) %>% 
  vis_dat(palette = "qual")
```

![](https://i.imgur.com/hMuPNxo.png)
