Update App.php

Good morning,

I wonder if you can take a look at the changes.
All changes are limited to App.php file and I pass all phpunit test-cases.
This is a description of the changes:
Get rid of the `_handleResponse()` method and rewrite `response()` method.
Additionally, create a `responseFactory()` method which is used only by `response()`.
Finally, there are some changes in the `run()` method too.

When I was trying to understand the logic, I get confused, there were several places that create/setup a response.
With these changes all response setup is contained inside the `responseFactory()` method. `response()` method is keep for compatibility reasons but it's only a a wrapper for `responseFactory()`. 
Finally the core functionality of `_handleResponse()` is moved to the end of `run()` method.

Hope you approve the changes. Cheers!