pandas.core.groupby.SeriesGroupBy.hist doesn't propagate **kwargs
When calling `my_groupby.hist(bins=8, histtype='stepfilled')` I would expect the same behavior as when calling `my_series.hist(bins=8, histtype='stepfilled')` separately for each of the groups.
However, while the 'bins' parameter is taken into account, the 'histtype' parameter is ignored and not passed along to matplotlib's implementation.

Looking into the code, it appears that `pandas.core.groupby._whitelist_method_generator` only passes along named arguments of the wrapped method (hist in our example). While the signature was fixed to include any `*args` or `**kwargs` arguments (done to fix #8733), they are not passed on.

Note: the problem applies to all 'wrappers' generated by _whitelist_method_generator (in addition to 'hist'), and also to any use of `*args` (in addition to `**kwargs`).

We are working on a PR and will send it soon.
#### output of `pd.show_versions()`

commit: None
python: 2.7.12.final.0
python-bits: 64
OS: Darwin
OS-release: 15.6.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8

pandas: 0.18.1
nose: 1.3.7
pip: 8.1.2
setuptools: 24.0.0
Cython: 0.23.4
numpy: 1.11.1
scipy: 0.17.1
statsmodels: 0.6.1
xarray: None
IPython: 4.0.1
sphinx: 1.3.1
patsy: 0.4.0
dateutil: 2.5.3
pytz: 2016.6.1
blosc: None
bottleneck: 1.0.0
tables: 3.2.2
numexpr: 2.5.2
matplotlib: 1.5.1
openpyxl: 2.2.6
xlrd: 0.9.4
xlwt: 1.0.0
xlsxwriter: 0.7.7
lxml: 3.4.4
bs4: 4.4.1
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.9
pymysql: None
psycopg2: None
jinja2: 2.8
boto: 2.38.0
pandas_datareader: None
