Simple string argument is Converted to integer because input only contains numeric data

A simple argument is automatically converted to integer because the input only has numeric data in a string. I wondered if it is a wanted feature. Thank you

Python3 version = Pyton3.8.10
Fire version = 0.5

file sample.py
```Python3
import fire
from fire import Fire

print(fire.__version__)

def sample(sequence:str):
print(sequence, type(sequence))

Fire(sample)
```
```bash
$ python3 ./sample.py "00"
0.5.0
0
```

Expected behavior: "00" string type

使用 sample.py 範例和命令 `python3 ./sample.py "00"` 重現轉換流程。追蹤 Python Fire 的 CLI 引數解析和轉換邏輯，以找出為什麼這個值會變成整數；當能夠將 "00" 保留為字串，並為回報的行為新增涵蓋測試時，即視為完成。

由索引模型根據 Issue 內容生成。