color attribute of medianprops is not correctly understand in a boxplot
#### Code Sample, a copy-pastable example if possible

```python
df = pd.DataFrame({k: np.random.random(100) for k in "ABC"})

# wrong output
df.boxplot(medianprops=dict(color="C1", lw=2))

# right output
plt.boxplot(df.transpose(), medianprops=dict(color="C1", lw=2))
```
#### Problem description

The problem comes from the boxplot function in a dataframe. This function is linked to the matplotlib function. I got an issue when I try to change the color of the medians using the `medianprops` argument of the function (which is a matplotlib argument). If I use the same instruction directly with the boxplot function in matplotlib I got the right behavior. But, I can change the `linewidth` or `linestyle` for example.

Here I put a screenshot of what I obtain if I use the `boxplot` method from pandas or directly from matplotlib. You can see that using the matplotlib function, the color of the median line is "correct" (the one I asked) while you cannot change the color of the line from the pandas function.

![Capture d’écran 2019-12-19 à 11 04 32](https://user-images.githubusercontent.com/5417018/71164774-fe27bf80-224f-11ea-87ff-5714faece96d.png)

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

<details>
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.4.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 19.0.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : fr_FR.UTF-8
LOCALE           : fr_FR.UTF-8

pandas           : 0.25.1
numpy            : 1.17.2
pytz             : 2019.3
dateutil         : 2.8.0
pip              : 19.2.3
setuptools       : 41.4.0
Cython           : 0.29.13
pytest           : 5.2.1
hypothesis       : None
sphinx           : 2.2.0
blosc            : None
feather          : None
xlsxwriter       : 1.2.1
lxml.etree       : 4.4.1
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.10.3
IPython          : 7.8.0
pandas_datareader: None
bs4              : 4.8.0
bottleneck       : 1.2.1
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.4.1
matplotlib       : 3.1.1
numexpr          : 2.7.0
odfpy            : None
openpyxl         : 3.0.0
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : 1.3.1
sqlalchemy       : 1.3.9
tables           : 3.5.2
xarray           : None
xlrd             : 1.2.0
xlwt             : 1.3.0
xlsxwriter       : 1.2.1
</details>
