fix dojo#886 ColumnHider on OSX disappeared, with latest css changes

We were adding the dgrid-header class to the headerScrollNode, which
will hide the ColumnHider icon on OSX browsers that hide the scrollbars
by default, because we set overflow:hidden in dgrid-header.

This is regression that appeared in v0.3.14, it was not present in
v0.3.13.

List.js

@@ -256,7 +256,7 @@ function(kernel, declare, dom, listen, has, miscUtil, TouchScroll, hasClass, put
 				bodyNode.tabIndex = -1;
 			}
 
-			this.headerScrollNode = put(domNode, "div.dgrid-header.dgrid-header-scroll.dgrid-scrollbar-width" +
+			this.headerScrollNode = put(domNode, "div.dgrid-header-scroll.dgrid-scrollbar-width" +
 				(addUiClasses ? ".ui-widget-header" : ""));
 
 			// Place footer node (initially hidden if showFooter is false).