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

Start at FluentRecordFormatter._structuring and reproduce the failure by attaching a fluent handler, then calling logger.info('1'). Confirm the behavior for JSON scalars and lists, and consider the issue done when these messages no longer raise AttributeError while dictionary messages continue to work.

索引モデルが issue の本文から書いたものです。