step option does not work for hist
#### Code Sample, a copy-pastable example if possible

```python
import matplotlib.pyplot as plt
import pandas as pd
import numpy as np
N = 1000
df = pd.DataFrame({
  'a': np.random.randint(0, 2, N),
  'b': np.random.random(N)
})
plt.figure()
df.groupby('a').hist(histtype='step', ax=plt.gca())
plt.legend()
plt.show()
```

This is what I get:
![image](https://user-images.githubusercontent.com/8983713/49219292-7d471d00-f3db-11e8-9b7a-a9d6c67c2dcf.png)

#### Problem description

This function should create a step hist which is unfilled

>     - 'step' generates a lineplot that is by default
>       unfilled.
> 

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.15.final.0
python-bits: 64
OS: Linux
OS-release: 4.14.65-gentoo
machine: x86_64
processor: Intel(R) Core(TM) i9-7900X CPU @ 3.30GHz
byteorder: little
LC_ALL: None
LANG: en_IL.US-ASCII
LOCALE: None.None

pandas: 0.23.4
pytest: 3.8.0
pip: 10.0.1
setuptools: 40.2.0
Cython: 0.28.5
numpy: 1.15.1
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 5.8.0
sphinx: 1.7.9
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.5
blosc: None
bottleneck: 1.2.1
tables: 3.4.4
numexpr: 2.6.8
feather: None
matplotlib: 2.2.3
openpyxl: 2.5.6
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.1.0
lxml: 4.2.5
bs4: 4.6.3
html5lib: 1.0.1
sqlalchemy: 1.2.11
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>