make 'pie' a layout category,

- so that fullLayout.has('pie') returns true
- note that fullLayout.has('cartesian') also returns true now
  as pie trace use the cartesian plot module.

Author: etpinard

src/plots/plots.js

@@ -193,7 +193,7 @@ plots.getSubplotIds = function getSubplotIds(layout, type) {
     if(type === 'cartesian' && !layout._hasCartesian) return [];
     if(type === 'gl2d' && !layout._hasGL2D) return [];
     if(type === 'cartesian' || type === 'gl2d') {
-        return Object.keys(layout._plots);
+        return Object.keys(layout._plots || {});
     }
 
     var idRegex = _module.idRegex,

src/traces/pie/index.js

@@ -23,6 +23,7 @@ Pie.moduleType = 'trace';
 Pie.name = 'pie';
 Pie.basePlotModule = require('../../plots/cartesian');
 Pie.categories = ['pie', 'showLegend'];
+Pie.layoutCategories = ['pie'];
 Pie.meta = {
     description: [
         'A data visualized by the sectors of the pie is set in `values`.',