First draw has tune=True sampler stat

## Description
The first draw after the tuning is marked with a `tune=True` sampler stat.

![grafik](https://user-images.githubusercontent.com/5894642/75537640-b2e01900-5a17-11ea-93f8-1a77cfd545b3.png)

## Versions and main components
* PyMC3 Version: master

Other components are not relevant here.