Fix TreeWalker's nextNode()

"Following" does not work here.

See https://www.w3.org/Bugs/Public/show_bug.cgi?id=28107 and #87 (comment).

dom.bs

@@ -9287,10 +9287,23 @@ steps:
      {{TreeWalker/currentNode}} attribute to <var>node</var> and return <var>node</var>.
     </ol>
 
-   <li><p>If there is a <a>node</a> that is <a>following</a> <var>node</var> and is not
-   <a>following</a> <a for=traversal>root</a>, then set <var>node</var> to the first such
-   <a>node</a>. Otherwise, return null.
-   <!-- Implemented as iterating over parent/nextSibling -->
+   <li><p>Let <var>sibling</var> be null.
+
+   <li><p>Let <var>temporary</var> be <var>node</var>.
+
+   <li>
+    <p>While <var>temporary</var> is non-null:
+
+    <ol>
+     <li><p>If <var>temporary</var> is the <a>context object</a>'s <a for=traversal>root</a>, then
+     return null.
+
+     <li><p>Set <var>sibling</var> to <var>temporary</var>'s <a for=tree>next sibling</a>.
+
+     <li><p>If <var>sibling</var> is non-null, then <a for=iteration>break</a>.
+
+     <li><p>Set <var>temporary</var> to <var>temporary</var>'s <a for=tree>parent</a>.
+    </ol>
 
    <li><p>Set <var>result</var> to the result of <a for=Node>filtering</a> <var>node</var>.