always compute guessed postcode for POIs from centroid

When guessing postcodes from the area, only postcodes within
that area are accepted. For POIs that is usually not what we
want as the postcode would have to be within a house for
example.

Fixes #2301.

lib-sql/functions/placex_triggers.sql

@@ -911,7 +911,7 @@ BEGIN
       -- determine postcode
       NEW.postcode := coalesce(token_normalized_postcode(NEW.address->'postcode'),
                                location.postcode,
-                               get_nearest_postcode(NEW.country_code, NEW.geometry));
+                               get_nearest_postcode(NEW.country_code, NEW.centroid));
 
       IF NEW.name is not NULL THEN
           NEW.name := add_default_place_name(NEW.country_code, NEW.name);