when using init_app still set self.app

I noticed that when using init_app the `self.app` was no longer being set.

You don't seem to use it except to access `app.extensions['cache']` but some people (like me) might overload your class and then it's nice to have `self.app` available.

If you disagree I guess you should remove `self.app` completely since imo the Cache(app) behavory should be exactly the same as when using init_app.