document.registerElement option argument should be optional

Fixes #406

src/wrappers/Document.js

@@ -110,7 +110,15 @@
   if (document.registerElement) {
     var originalRegisterElement = document.registerElement;
     Document.prototype.registerElement = function(tagName, object) {
-      var prototype = object.prototype;
+      var prototype, extendsOption;
+      if (object !== undefined) {
+        prototype = object.prototype;
+        extendsOption = object.extends;
+      }
+
+      if (!prototype)
+        prototype = Object.create(HTMLElement.prototype);
+
 
       // If we already used the object as a prototype for another custom
       // element.
@@ -171,13 +179,13 @@
       });
 
       var p = {prototype: newPrototype};
-      if (object.extends)
-        p.extends = object.extends;
+      if (extendsOption)
+        p.extends = extendsOption;
 
       function CustomElementConstructor(node) {
         if (!node) {
-          if (object.extends) {
-            return document.createElement(object.extends, tagName);
+          if (extendsOption) {
+            return document.createElement(extendsOption, tagName);
           } else {
             return document.createElement(tagName);
           }

test/js/Document.js

@@ -357,7 +357,7 @@ htmlSuite('Document', function() {
     };
 
     var A = document.registerElement('x-a-span',
-      {prototype: aPrototype, extends: 'span'});
+        {prototype: aPrototype, extends: 'span'});
 
     var a1 = document.createElement('span', 'x-a-span');
     assert.equal('span', a1.localName);
@@ -554,6 +554,28 @@ htmlSuite('Document', function() {
     assert.isTrue(div.firstChild.isCustom);
   });
 
+  test('document.registerElement optional option', function() {
+    if (!document.registerElement)
+      return;
+
+    document.registerElement('x-a7');
+    var a = document.createElement('x-a7');
+    assert.equal(Object.getPrototypeOf(Object.getPrototypeOf(a)),
+                 HTMLElement.prototype);
+
+    document.registerElement('x-a8', {});
+    var a2 = document.createElement('x-a8');
+    assert.equal(Object.getPrototypeOf(Object.getPrototypeOf(a2)),
+                 HTMLElement.prototype);
+
+    document.registerElement('x-a-span-2', {extends: 'span'});
+    var a3 = document.createElement('span', 'x-a-span-2');
+    assert.equal(Object.getPrototypeOf(Object.getPrototypeOf(a3)),
+                 HTMLElement.prototype);
+    a3.localName = 'span';
+    assert.equal('<span is="x-a-span-2"></span>', a3.outerHTML);
+  });
+
   htmlTest('html/document-write.html');
 
   htmlTest('html/head-then-body.html');