Autodetection of date format is confused by underscores

[krlmlr]

> 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?

[hadley]

This is related to #463