Make `abbreviations` argument work with case = `"none"`

I.e. this should return "catcatdog"

```r
> x <- "CatCATdog"
> to_any_case(x, abbreviations = c("Cat" = "cat"))
[1] "cat_cat_dog"
> to_any_case(x, abbreviations = c("Cat" = "cat"), case = "none")
[1] "CatCATdog"
```