Ability to Unnest Series of Dicts
#### Code Sample, a copy-pastable example if possible

```python
s = pd.Series([{'a': {'b': 1, 'c': 2}}, {'a': {'b': 3, 'c': 4}}], index=[1, 3])
s

1    {'a': {'b': 1, 'c': 2}}
3    {'a': {'b': 3, 'c': 4}}
dtype: object


json_normalize(s)

   a.b  a.c
0    1    2
1    3    4
```
#### Problem description

Maybe I'm missing some nuance of nesting or something that would potentially result in a different number of rows...? But it seems like if the rows of the output correspond to those of the input then the index should match as well.

#### Output of ``pd.show_versions()``

`pandas==0.25.3`