Don't store is value when it won't be useful

Fixes #566.

dom.bs

@@ -6052,6 +6052,15 @@ given a <var>document</var>, <var>localName</var>, <var>namespace</var>, and opt
    <li><p>Let <var>interface</var> be the <a>element interface</a> for <var>localName</var> and
    <var>namespace</var>.
 
+   <li>
+    <p>If <var>localName</var> is a <a>valid custom element name</a>, or <var>is</var> is not a
+    <a>valid custom element name</a>, then set <var>is</var> to null.</p>
+
+    <p class="note">In this case, even though <var>is</var> was supplied, there's no way that this
+    element will ever be <a lt="upgrade an element">upgraded</a>, so we shouldn't store the
+    <a for=Element><code>is</code> value</a>.</p>
+   </li>
+
    <li><p>Set <var>result</var> to a new <a for=/>element</a> that implements <var>interface</var>,
    with no attributes, <a for=Element>namespace</a> set to <var>namespace</var>,
    <a for=Element>namespace prefix</a> set to <var>prefix</var>, <a for=Element>local name</a> set