fixed Polygon points default attr issue

src/shapes/Polygon.js

@@ -41,16 +41,17 @@
          */
         setPoints: function(val) {
             this.setAttr('points', Kinetic.Type._getPoints(val));
-        }
-    };
-    Kinetic.Global.extend(Kinetic.Polygon, Kinetic.Shape);
-
-    // add getters setters
-    Kinetic.Node.addGetter(Kinetic.Polygon, 'points', []);
-
+        },
         /**
          * get points array
          * @name getPoints
          * @methodOf Kinetic.Polygon.prototype
          */
+         // NOTE: cannot use getter method because we need to return a new
+         // default array literal each time because arrays are modified by reference
+        getPoints: function() {
+            return this.attrs.points || [];
+        }
+    };
+    Kinetic.Global.extend(Kinetic.Polygon, Kinetic.Shape);
 })();