adds test on multipolygon

src/pg/test/expected/13_pia_test.out

@@ -10,3 +10,8 @@ SET client_min_messages TO WARNING;
  POINT(0 0)
 (1 row)
 
+ st_astext  
+------------
+ POINT(0 0)
+(1 row)
+

src/pg/test/sql/13_pia_test.sql

@@ -11,5 +11,16 @@ SELECT st_astext(cdb_crankshaft.CDB_PIA(g)) from a;
 WITH square AS (
   SELECT 'SRID=4326;POLYGON((-1 1, 1 1, 1 -1, -1 -1, -1 1))'::geometry as g
 )
+SELECT ST_AsText(cdb_crankshaft.CDB_PIA(g))
+  FROM square;
+
+-- MultiPolygon test
+-- square centered on 0,0 with sides of length 2
+-- expectation: point(0, 0)
+WITH square AS (
+  SELECT
+    ST_Multi('SRID=4326;POLYGON((-1 1, 1 1, 1 -1, -1 -1, -1 1))'::geometry) as g
+)
 SELECT ST_AsText(cdb_crankshaft.CDB_PIA(g))
   FROM square
+