[BUG] Fix behavior of SARIMAXModel if simple_differencing=True is set

[Mr-Geekman]

🐛 Bug Report

According to documentation there is a problem with simple_differencing=True option:

It means that if we use this option then our results aren't correct and should be undifferenced back. We should make a fix that make this undifferencing.

We can use pmdarima solution as a reference or import this function.

We should also add tests that solution with simple_differencing=True and without it are close.

Expected behavior

Simple differencing works fine.

How To Reproduce

Look at problem above.

Environment

No response

Additional context

No response

Checklist

• Bug appears at the latest library version