pandas.DataFrame.rolling.apply Can't deal with np.nan
#### Code Sample, a copy-pastable example if possible

```python
# Your code here
import pandas as pd
import numpy as np

df = pd.DataFrame({'a':[1,2,3,4,5], 'b':[10,9,8,7,np.nan], 'c':[8,7,6,5,4], 'd':[4,np.nan,6,7,8]})
df

def sum_(data):
    return data.sum()
res= df.rolling(3,center=False).apply(sum_,raw=True)
res
```
#### Problem description

[pd.DataFrame.rolling.apply    Can't deal with np.nan]
