Fix bug "Failed to execute 'insertAdjacentHTML' on 'Element'"

Due to undefined value for where, which didn't used to be required.

inst/www/shared/shiny.js

@@ -3122,6 +3122,7 @@ var _typeof = typeof Symbol === "function" && typeof Symbol.iterator === "symbol
   var singletons = {
     knownSingletons: {},
     renderHtml: function renderHtml(html, el, where) {
+      where = where || "replace";
       var processed = this._processHtml(html);
       this._addToHead(processed.head);
       this.register(processed.singletons);

srcjs/output_binding_html.js

@@ -136,6 +136,7 @@ function renderDependency(dep) {
 var singletons = {
   knownSingletons: {},
   renderHtml: function(html, el, where) {
+    where = where || "replace";
     var processed = this._processHtml(html);
     this._addToHead(processed.head);
     this.register(processed.singletons);