API: should allow setting with enlargement with list-likes
from #7867 

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

In [2]: s
Out[2]: 
0    1
1    2
2    3
dtype: int64

# this is de-facto: s.reindex([[1,2,3]])
In [3]: s.loc[[1,2,3]]
Out[3]: 
1     2
2     3
3   NaN
dtype: float64
```

should this work?

```
In [4]: s.loc[[1,2,3]] = 4
KeyError: '[3] not in index'
```

(Doing this with only included labels works)
