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 commit is a very silly workaround for it.

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.