fix too early unmount

[szyablitsky]

This change is

[justin808]

@szyablitsky This is super awesome! Would you like to create a failing without, passing with test in the spec/dummy app? If not, I can try. I will merge your commit into a branch on my machine and then I'll push up a new PR. If you can write a test, that would be AMAZING! Thanks so much!

[szyablitsky]

@justin808 testing this can be very tricky because of Capybara's waiting behavior. https://github.com/jnicklas/capybara#asynchronous-javascript-ajax-and-friends
Also rerendering actually happens every time. But depending on cache state, time between turbolinks:before-cache and turbolinks:before-render can be zero (in which case rerendering is not visible) or it can be big enough to see the empty space in place of unmounted component.

[justin808]

@szyablitsky At the least, we can put in an example that confirms this manually, in spec/dummy

I'm also going to push an RC with this fix and test it in my branch for the https://github.com/shakacode/react-webpack-rails-tutorial/.

[justin808]

I merged this change here:

40553fc

We should still have an example of this in spec/dummy.