Make the DOMTokenList creation steps use "get an attribute value"

This makes it so that if a DOMTokenList's associated element has an attribute that is the associated attribute's local name, at creation time, it will use the "get an attribute value" steps, which will always return a string. Previously, the this used the "get an attribute by namespace and local name" steps, which would return an Attr node if the associated element had an attribute that is the associated attribute's local name.

Fixes #666.

dom.bs

@@ -9525,8 +9525,8 @@ are to return the result of running <a>get an attribute value</a> given the asso
  <li><p>Let <var>localName</var> be associated attribute's <a for=Attr>local name</a>.
 
  <li><p>Let <var>value</var> be the result of
- <a lt="get an attribute by namespace and local name">getting an attribute</a> given null,
- <var>localName</var>, and <var>element</var>.
+ <a lt="get an attribute value">getting an attribute value</a> given <var>element</var> and
+ <var>localName</var>.
 
  <li><p>Run the <a>attribute change steps</a> for <var>element</var>, <var>localName</var>,
  <var>value</var>, <var>value</var>, and null.
@@ -10026,6 +10026,7 @@ Timo Tijhof,
 Tobie Langel,
 Tom Pixley,
 Travis Leithead,
+Trevor Rowbotham,
 <i>triple-underscore</i>,<!--GitHub-->
 Veli Şenol,
 Vidur Apparao,