Change in behaviour of unique method regarding None values
The behaviour of `unique` regarding `None` values recently changed on master (somewhere in the last month) :

```
In [1]: pd.__version__
Out[1]: '0.22.0'

In [2]: s = pd.Series(['test', None])

In [3]: s.unique()
Out[3]: array(['test', None], dtype=object)
```

vs

```
In [1]: pd.__version__
Out[1]: '0.23.0.dev0+807.g563a6ad'

In [2]: s = pd.Series(['test', None])

In [3]: s.unique()
Out[3]: array(['test', nan], dtype=object)
```

Not sure what the desired result is (we typically handle None like it is missing like NaN, but on the other hand, we also didn't coerce to it on construction, i.e. we do allow None values to be stored), but it is a breaking change (discovered it because geopandas tests started failing; https://github.com/geopandas/geopandas/issues/711).