Sums not treating NaN as zero.
Simple example:

```
In [136]: dfa
Out[136]: 
     clicks
ad         
327       3

In [137]: dfc
Out[137]: 
     clicks
ad         
326       1

In [138]: dfa + dfc
Out[138]: 
     clicks
ad         
326     NaN
327     NaN

#Expected
In [138]: dfa + dfc
Out[138]: 
     clicks
ad         
326     1
327     3
```

Per docs, NaN should be treated as 0 unless all values are NaN.
