Verifying state in Turbolinks.visit

More than once, after visiting lots of URLs with Turbolinks.visit,
most of times changing the location.search, the window.history.state
became null for some reason.

This PR commit contains a very silly workaround for it, since I can't find any other solution.

The issue is similar as the described [here](https://github.com/rails/turbolinks/pull/199#issuecomment-19731190).

I could reproduce it in Chrome 27 in OSX.