separate convert with stringsAsFactors = FALSE

Hi, I know that you are not a big fan of stringsAsFactors. However 

``` r
df <- data.frame(x = c("a.b", "a.d", "b.c"))
df %>% 
    separate(x, c("A", "B"), convert = TRUE) %>% 
    str()
'data.frame':   3 obs. of  2 variables:
 $ A: Factor w/ 2 levels "a","b": 1 1 2
 $ B: Factor w/ 3 levels "b","c","d": 1 3 2
```

Would it be a good idea to send stringsAsFactors = FALSE to `separate`?
