If you pass an empty list, tuple, False, 0, whatever.. it will assume th
token is valid, which it isn't.

Currently the module raises generic Exception (very "don't do it")

Currently logging with:
```
logger.info('hello %s', 'world')
```
will send the string "hello %s" to logz.io instead of "hello world"

This commit fixes this situation.

Add tests

Allow custom logzio fields to top level structure