df.sort_values() not respecting na_position with categoricals
#### Problem description
`DataFrame.sort_values()` appears not to respect the `na_position` parameter when sorting by a categorical series:

```python
>>> import pandas as pd
>>> c = pd.Categorical(['A', np.nan, 'B'], categories=['A','B'], ordered=True)
>>> df = pd.DataFrame({'c': c})
>>> df.sort_values(by='c', na_position='first')
     c
1  NaN
0    A
2    B
>>> df.sort_values(by='c', na_position='last')
     c
1  NaN
0    A
2    B
```
Unexpectedly, the NaNs always come first regardless of `na_position`.

#### Additional information
`Series.sort_values()` works as expected:
```python
>>> c.sort_values(na_position='first')
[NaN, A, B]
Categories (2, object): [A < B]
>>> c.sort_values(na_position='last')
[A, B, NaN]
Categories (2, object): [A < B]
```

Strangely, `df.sort_values()` does seem to respect `na_position` if you sort by more than one column (even the same column):
```python
>>> df.sort_values(by=['c','c'], na_position='first')
     c
1  NaN
0    A
2    B
>>> df.sort_values(by=['c','c'], na_position='last')
     c
0    A
2    B
1  NaN
```

#### Output of ``pd.show_versions()``

<details>

```
INSTALLED VERSIONS
------------------
commit: None
python: 3.7.0.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 94 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.23.4
pytest: None
pip: 10.0.1
setuptools: 40.0.0
Cython: None
numpy: 1.15.0
scipy: None
pyarrow: None
xarray: None
IPython: 6.5.0
sphinx: None
patsy: None
dateutil: 2.7.3
pytz: 2018.5
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
```
</details>
