Use valid css selector for input[name=...

As of Capybara 2.1 css selectors are sent straight through to
the driver.
https://github.com/jnicklas/capybara/blob/master/History.md#added
Find hidden input field finders
Merge pull request #109 from spedmg/fix-input-css-selectors

Use valid css selector for input[name=...