Make node#set work for select elements

Capybara doesn't seem to specify exactly what should happen when you `set` a "SELECT" element but the capybara-webkit driver honors the request. This is a nice feature to have when using a page object framework like site_prism which doesn't distinguish between elements of different types and encourages you to set them all the same way.

This pull request makes poltergeist behave the same as webkit in this respect.