Avoid to render or redirect multiple times

This PR fix this error:

AbstractController::DoubleRenderError (Render and/or redirect were called multiple times in this action. Please note that you may only call render OR redirect, and at most once per action. Also note that neither redirect nor render terminate execution of the action, so if you want to exit an action after redirecting, you need to do something like "redirect_to(...) and return".):

When tried to request a AMP page but the app redirects or render before is raising that error.

I think that the best approach is to check if render or redirect has already been called before.

takafumir / rails_amp

Avoid to render or redirect multiple times #2