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