Reverse operators on integer-NA series and numpy scalars resulting in object dtype
This is not necessarily a bug, but at least some unexpected results:

```
In [83]: s = pd.Series([1, 2, 3], dtype='Int64')

In [84]: s + s[0]
Out[84]: 
0    2
1    3
2    4
dtype: Int64

In [85]: s[0] + s
Out[85]: 
0    2
1    3
2    4
dtype: object
```

In the above `s[0]` is a numpy scalar, if you do a `radd` with a normal int, you do not get object:

```
In [86]: 1 + s
Out[86]: 
0    2
1    3
2    4
dtype: Int64

In [87]: type(s[0])
Out[87]: numpy.int64
```
