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 `cf3f10e` demonstrates this behavior with a test, `3813ca9` fixes it. ActiveRecord test suite is otherwise unaffected by the change.

This bug was observed as a regression from `3.2.13` to `3.2.16`.