Dataframe Groupby value_counts with bins parameter
Found on Stack Overflow post [here](https://stackoverflow.com/q/60553500/6361531)
```python
df=pd.DataFrame([[0,0],[1,100],[0,100],[2,0],[3,100],[4,100],[4,100],[4,100],[1,100],[3,100]],columns=['key','score'])
df.groupby('key')['score'].value_counts(bins=[0,20,40,60,80,100])
```
#### Problem description

Outputs counts at the with wrong index labels.  **_Note:_** data only occurs in 0-20 and 80-100.
```
key  score         
0    (-0.001, 20.0]    1
     (20.0, 40.0]      1
     (40.0, 60.0]      0
     (60.0, 80.0]      0
     (80.0, 100.0]     0
1    (20.0, 40.0]      2
     (-0.001, 20.0]    0
     (40.0, 60.0]      0
     (60.0, 80.0]      0
     (80.0, 100.0]     0
2    (-0.001, 20.0]    1
     (20.0, 40.0]      0
     (40.0, 60.0]      0
     (60.0, 80.0]      0
     (80.0, 100.0]     0
3    (20.0, 40.0]      2
     (-0.001, 20.0]    0
     (40.0, 60.0]      0
     (60.0, 80.0]      0
     (80.0, 100.0]     0
4    (20.0, 40.0]      3
     (-0.001, 20.0]    0
     (40.0, 60.0]      0
     (60.0, 80.0]      0
     (80.0, 100.0]     0
Name: score, dtype: int64
```
#### Expected Output

`df.groupby('key')['score'].apply(pd.Series.value_counts, bins=[0,20,40,60,80,100])`
```
key                
0    (80.0, 100.0]     1
     (-0.001, 20.0]    1
     (60.0, 80.0]      0
     (40.0, 60.0]      0
     (20.0, 40.0]      0
1    (80.0, 100.0]     2
     (60.0, 80.0]      0
     (40.0, 60.0]      0
     (20.0, 40.0]      0
     (-0.001, 20.0]    0
2    (-0.001, 20.0]    1
     (80.0, 100.0]     0
     (60.0, 80.0]      0
     (40.0, 60.0]      0
     (20.0, 40.0]      0
3    (80.0, 100.0]     2
     (60.0, 80.0]      0
     (40.0, 60.0]      0
     (20.0, 40.0]      0
     (-0.001, 20.0]    0
4    (80.0, 100.0]     3
     (60.0, 80.0]      0
     (40.0, 60.0]      0
     (20.0, 40.0]      0
     (-0.001, 20.0]    0
Name: score, dtype: int64
```



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

<details>

[paste the output of ``pd.show_versions()`` here below this line]
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.3.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 63 Stepping 2, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None

pandas           : 1.0.1
numpy            : 1.16.4
pytz             : 2019.1
dateutil         : 2.8.0
pip              : 19.1.1
setuptools       : 41.0.1
Cython           : 0.29.12
pytest           : 5.0.1
hypothesis       : None
sphinx           : 2.1.2
blosc            : None
feather          : None
xlsxwriter       : 1.1.8
lxml.etree       : 4.3.4
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.10.1
IPython          : 7.7.0
pandas_datareader: None
bs4              : 4.7.1
bottleneck       : 1.2.1
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.3.4
matplotlib       : 3.1.0
numexpr          : 2.6.9
odfpy            : None
openpyxl         : 2.6.2
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : 5.0.1
pyxlsb           : None
s3fs             : None
scipy            : 1.3.0
sqlalchemy       : 1.3.5
tables           : 3.5.2
tabulate         : 0.8.6
xarray           : None
xlrd             : 1.2.0
xlwt             : 1.3.0
xlsxwriter       : 1.1.8
numba            : 0.45.0
</details>
