agate: Inappropriate cast to a date

Created by: fgregg

csvkit==1.0.2, ubuntu

The following one column csv casts the "comment" field to a date, inappropriately (this is a reduced test case):

COMMENT
MC 5.7.10 Per Dorothy Carroll
testing workgroup fix - 4/7/2010 - Marcy Liberty

> printf "COMMENT\nMC 5.7.10 Per Dorothy Carroll\ntesting workgroup fix - 4/7/2010 - Marcy Liberty" | csvlook
|    COMMENT |
| ---------- |
| 2010-05-07 |
| 2010-04-07 |