Fix inconsistency in ActiveRecord::ConnectionAdapters::Column's handing of two-digit dates

ActiveRecord currently converts two-digit non ISO date strings to `:datetime`, but fails when converting to `:date`. Commit `7ef3bc7` demonstrates this behavior with a test, `3763cb2` fixes it. ActiveRecord test suite is otherwise unaffected by the change.

This bug is also present in `3.2-stable`, discovered while upgrading from 3.2.13 -> 3.2.16.