When the postgis tests are done, I will close mapbox/carto#75
Followup to #1642 and #794
also need to review entire behavior after 562fada#include/mapnik/feature.hpp, in that we may no longer need to return actual value_null() instance from datasource plugins.
add null and boolean comparison test for postgis - refs #794, #1642, #…
confirmed that for postgres we do not need to store null values. storing nulls is equivalent to accessing attributes that do not exist in the feature, but saves space. The only other plugins that started storing nulls were sqlite and shape and they should behave the same.
plugins do not need to store nulls / save space - refs #1644