Autodetection of date format is confused by underscores

```
> readr::read_csv("a\n2_15_3")
Warning: 1 parsing failure.
row col   expected actual
  1   a valid date 2_15_3
# A tibble: 1 x 1
       a
  <date>
1   <NA>
```

Why does readr think `"2_15_3"` is a date?
