postgresql_adaptor.rb author: Luke Holden <lholden@cablelan.net> notes: Currently this adaptor does not pass the test_zero_date_fields

       and test_zero_datetime_fields unit tests in the BasicsTest test
       group.

       This is due to the fact that, in postgresql you can not have a
       totally zero timestamp. Instead null/nil should be used to
       represent no value.