FluentRecordFormatter._structuring may raise AttributeError

`json.loads(str(msg))` may not return a dict, it can return:
- int if `msg` is a json dumped from an int, for example `msg = '123'`
- list if `msg` is a json dumped from a list, for example `msg = '["s", "1"]'`

Only dict have items() method.

So if you get a logger and add a fluent.handler to it, then you execute
`logger.info('1')`
you will get an AttributeError

從 FluentRecordFormatter._structuring 開始，透過附加一個 fluent handler，然後呼叫 logger.info('1') 來重現該故障。確認 JSON scalar 和 list 的行為，並在這些訊息不再引發 AttributeError，而 dictionary 訊息繼續正常運作時，視為該 issue 已完成。

由索引模型根據 Issue 內容生成。