Fix rendering of selected options in tags.select() helper.

I updated the ``html.tags.select()`` helper to correct the issue with selected options not being set properly, and removed the xfail mark from the tests. It's verified on py27 and py33.

I've also added a test for calling ``select()`` with boolean values-- not sure if that's intended to work or not, but it seems like something that people would try to do.

Please let me know if there's anything else I can do on this, and thanks for your work on WebHelpers2!