Setting histogram weights for multiple columns fails
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
import numpy as np
df = pd.DataFrame(dict(zip(['A', 'B'], np.random.randn(2, 100))))
df.plot.hist(weights=0.1*np.ones(shape=(100, 2)))  # fail
```
#### Problem description
Trying to plot a histogram of a multi-column data frame with weights fails with `ValueError: weights should have the same shape as a.` There is no error for a single column. It doesn't work with:
```python
df.plot.hist(weights=0.1*np.ones(shape=(100,)))  # fail
```
either.

#### Expected Output
A `matplotlib` figure.
![pandas_hist_weights_bug_01](https://user-images.githubusercontent.com/37543656/78025387-da493f00-7351-11ea-8b5f-a047dad14c9d.png)

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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.6.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 142 Stepping 9, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None
pandas           : 1.0.3
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 46.1.1.post20200323
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.1.3
numexpr          : None
odfpy            : None
openpyxl         : 3.0.3
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>
