Select: Add label class correctly when independent from browser DOM.

I found a issue about `ui-select` class of label with selectmenu in independent DOM tree.
Our project use jQuery mobile with Backbone.js, and create DOM tree independent from browser DOM dynamically.

If browser DOM contains label tag, add `ui-select` class to the label when call `selectmenu()` on select tag correctly.
But, `selectmenu()` doesn't add `ui-select` class when independent DOM contains label tag.

TestPage: http://jsbin.com/asilus/5/edit

I expect to set same class on the label each way.