Series Group-by Boxplot has Overlaid Plots
#### Code Sample

```python
import numpy as np
import pandas as pd

sr = pd.Series(index=pd.MultiIndex.from_product([[1, 2, 3], ['A', 'B', 'C', 'D', 'E']]), data=np.random.rand(15))
sr.groupby(level=0).plot.box()
```
#### Problem description
Below is an image of what currently occurs:
![series_boxplot_bad](https://user-images.githubusercontent.com/5217766/77718929-fd6da900-7048-11ea-95a8-33a1283bd409.png)
All three boxplots are plotted on top of each other in the same figure.

#### Expected Output
Either plot each group next to each other in the same figure, as in
```python
sr.unstack(0).plot.box()
```
![series_boxplot_good](https://user-images.githubusercontent.com/5217766/77719120-5d644f80-7049-11ea-99ee-7b22a6f4ab12.png)

and similarly as in
```python
df = pd.DataFrame(sr)
df.groupby(level=0).boxplot(subplots=False)
```
![dataframe_boxplot](https://user-images.githubusercontent.com/5217766/77719896-4c1c4280-704b-11ea-8674-ba764aa5e45e.png)


or create three separate figures instead of one, which is what happens in the `DataFrame` case:
```python
df = pd.DataFrame(sr)
df.groupby(level=0).plot.box()
```


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

<details>

```
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.8.2.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.5.13-arch1-1
machine          : x86_64
processor        : 
byteorder        : little
LC_ALL           : None
LANG             : en_NZ.UTF-8
LOCALE           : en_NZ.UTF-8

pandas           : 1.0.3
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 41.2.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.1
IPython          : 7.12.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.1.3
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : None
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : 0.48.0
```

</details>
