Hexbin plot ignores matplotlib default colormap
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
import matplotlib as mpl
mpl.rcParams['image.cmap'] = 'Greys'
df = pd.DataFrame(np.random.randn(1000, 2), columns=["a", "b"])
df["b"] = df["b"] + np.arange(1000)
df.plot.hexbin(x="a", y="b", gridsize=25)  # this is not gray...
```

#### Problem description

I don't know if this is intentional behaviour, but plotting a data frame as hexbin with matplotlib, the matplotlib default colormap (e.g. set by the style sheet, via `rcParams`, or whatever) is ignored due to this line:

https://github.com/pandas-dev/pandas/blob/master/pandas/plotting/_matplotlib/core.py#L1010

```python
cmap = self.colormap or "BuGn"
```

What I would expect as a user is that pandas uses the default colormap I have set rather than `"BuGn"`.

Simply changing this line to
```python
cmap = self.colormap
```
is enough (since `plt.cm.get_cmap(None)` will return the default color map). If you agree, I can submit a PR.

#### Expected Output

A gray hexbin plot.

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

<details>


INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.5.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.3.0-26-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : de_DE.UTF-8
LOCALE           : de_DE.UTF-8

pandas           : 1.0.0
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 19.3.1
setuptools       : 44.0.0
Cython           : 0.29.14
pytest           : 5.3.5
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              : 4.8.2
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.1.3
numexpr          : 2.7.1
odfpy            : None
openpyxl         : 3.0.3
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : 5.3.5
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : 1.3.13
tables           : 3.6.1
tabulate         : 0.8.6
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None

</details>
