Merge branch 'patch-4' of https://github.com/curran/d3-selection into curran-patch-4

Author: mbostock

src/selection/merge.js

@@ -1,6 +1,7 @@
 import {Selection} from "./index.js";
 
-export default function(selection) {
+export default function(context) {
+  var selection = context.selection ? context.selection() : context;
   if (!(selection instanceof Selection)) throw new Error("invalid merge");
 
   for (var groups0 = this._groups, groups1 = selection._groups, m0 = groups0.length, m1 = groups1.length, m = Math.min(m0, m1), merges = new Array(m0), j = 0; j < m; ++j) {