Ensure logging a non-object/non-string doesn't throw

This fixes the following exception:
```
> log.info(23)
TypeError: Object.keys called on non-object
    at Function.keys (native)
    at objCopy (/bunyan/lib/bunyan.js:54:16)
    at mkRecord (/bunyan/lib/bunyan.js:764:39)
    at Logger.info (/bunyan/lib/bunyan.js:795:19)
```

Not sure exactly what you want to do for `undefined`/`null`/`Function` types - I've just left them being converted without special cases - but they don't throw at least (check the attached test for details)