Regression: None in string input is wrongly cast to the "None" string
In Pandas 0.22.0:

```python
>>> df = pd.DataFrame({'data': ['x', None]}, dtype=str)
>>> df['data'].tolist()
['x', None]
```

In Pandas 0.23.0:
```python
>>> df = pd.DataFrame({'data': ['x', None]}, dtype=str)
>>> df['data'].tolist()
['x', 'None']
```

```python
>>> pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.5.final.0
python-bits: 64
OS: Linux
OS-release: 4.15.0-20-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: fr_FR.UTF-8
LOCALE: fr_FR.UTF-8

pandas: 0.23.0
pytest: 3.3.2
pip: 10.0.1
setuptools: 38.4.0
Cython: 0.28.2
numpy: 1.14.2
scipy: None
pyarrow: 0.9.1.dev14+g6599ab0.d20180323
xarray: None
IPython: 6.2.1
sphinx: 1.6.7
patsy: None
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.1.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
```