Fix for DataFrame.hist() with by- and weights-keyword
closes https://github.com/pydata/pandas/issues/9540

for example:

``` python
import pandas as pd
d = {'one' : ['A', 'A', 'B', 'C'],
     'two' : [4., 3., 2., 1.],
     'three' : [10., 8., 5., 7.]}     
df = pd.DataFrame(d)
df.hist('two', by='one', weights='three', bins=range(0, 10))
```

does not seem to break anything, but this is my first meddling in the pandas library, so a review would be nice
