How can I get json param

`test.py`
```
import fire

def hello(name):
print(name)
print(name.get('a'))

if __name__ == "__main__":
fire.Fire(hello)
```
example:
```
python test.py '{"a":"aaa","b":[1,2,3]}'
```
the param `name` is json dict, result print:
```
{'a':'aaa','b':[1,2,3]}
'aaa'
```
but when input:
```
python test.py '{"a":"aa.a","b":[1,2,3]}'
```
the param `name` can't parse to json, it's a `str`,how can I get the json param??
```
{a:aa.a,b:[1,2,3]}
```

Start with the test.py example and run both command lines shown in the issue to reproduce the differing argument parsing. Read the Python Fire argument-parsing path involved, then determine whether dotted JSON values should remain a dictionary; done requires either a confirmed fix with a regression test or a clear documented limitation.

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