Fixes OID warnings when using PostgreSQL

Postgres correctly identifies OIDs when BEARING and DISTANCE contain
numerical values. It fails to identify OID for a NULL type.

This patch explicitly sets an OID type for NULLs when running in
PostgreSQL environment.

lib/geocoder/stores/active_record.rb

@@ -45,7 +45,8 @@ module Geocoder::Store
             # If no lat/lon given we don't want any results, but we still
             # need distance and bearing columns so you can add, for example:
             # .order("distance")
-            select(select_clause(nil, "NULL", "NULL")).where(false_condition)
+            null_val = using_postgres? ? 'NULL::text' : 'NULL'
+            select(select_clause(nil, null_val, null_val)).where(false_condition)
           end
         }
 
@@ -64,7 +65,8 @@ module Geocoder::Store
               full_column_name(geocoder_options[:longitude])
             ))
           else
-            select(select_clause(nil, "NULL", "NULL")).where(false_condition)
+            null_val = using_postgres? ? 'NULL::text' : 'NULL'
+            select(select_clause(nil, null_val, null_val)).where(false_condition)
           end
         }
       end
@@ -224,6 +226,10 @@ module Geocoder::Store
         connection.adapter_name.match(/sqlite/i)
       end
 
+      def using_postgres?
+        connection.adapter_name.match(/postgres/i)
+      end
+
       ##
       # Value which can be passed to where() to produce no results.
       #