geom_polygon needs custom NA handling.

Fixes #1411

Author: hadley

R/geom-polygon.r

@@ -99,6 +99,10 @@ GeomPolygon <- ggproto("GeomPolygon", Geom,
   default_aes = aes(colour = "NA", fill = "grey20", size = 0.5, linetype = 1,
     alpha = NA),
 
+  handle_na = function(data, params) {
+    data
+  },
+
   required_aes = c("x", "y"),
 
   draw_key = draw_key_polygon