Sync execCommand() and queryCommand*() IDL with spec

LayoutTests/editing/execCommand/arguments-combinations.html

Index: LayoutTests/editing/execCommand/arguments-combinations.html
diff --git a/LayoutTests/editing/execCommand/arguments-combinations.html b/LayoutTests/editing/execCommand/arguments-combinations.html
index 3812c0e62a084c2f73e8c2d9e2fb86e773530717..9f5c112ea14cb1f6321347f420778ad1b102174f 100644
--- a/LayoutTests/editing/execCommand/arguments-combinations.html
+++ b/LayoutTests/editing/execCommand/arguments-combinations.html
@@ -69,10 +69,10 @@
                 // Test empty String 3rd parameter. (should not add an empty id value)
                 test('document.execCommand("InsertHorizontalRule", false, "")', '<hr>');
 
-                // Test null 3rd parameter. (should treat as if only two arguments were passed, same as undefined)
-                test('document.execCommand("InsertHorizontalRule", false, null)', '<hr>');
+                // Test null 3rd parameter. (should stringify to "null")
+                test('document.execCommand("InsertHorizontalRule", false, null)', '<hr id="null">');
 
-                // Test undefined 3rd parameter. (should treat as if only two arguments were passed, same as null)
+                // Test undefined 3rd parameter. (should treat as if only two arguments were passed)
                 test('document.execCommand("InsertHorizontalRule", false, undefined)', '<hr>');
 
                 // Test 0 for 3rd parameter. (nothing special, id value should equal the string 0)