Guard DOMImplIE6.hasAttribute(name) against returning undefined, which IE8 apparently can do.

Review by: jgw, jlabanca

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@5535 8db76d5a-ed1c-0410-87a9-c151d255dfc7
diff --git a/user/src/com/google/gwt/dom/client/DOMImplIE6.java b/user/src/com/google/gwt/dom/client/DOMImplIE6.java
index 98dee7f..f0581be 100644
--- a/user/src/com/google/gwt/dom/client/DOMImplIE6.java
+++ b/user/src/com/google/gwt/dom/client/DOMImplIE6.java
@@ -45,7 +45,7 @@
 
   public native boolean hasAttribute(Element elem, String name) /*-{
     var node = elem.getAttributeNode(name);
-    return node && node.specified;
+    return !!(node && node.specified);
   }-*/;
 
   /*