Add tests of treatment of NULL values

I was a bit surprised by the treatment of `NULL` values.

I thought it'd be useful to document the behavior in some tests.

I included the case that @yihui [tweeted](https://twitter.com/xieyihui/status/462342867818532864), in which the latest version of [RJSONIO](https://github.com/duncantl/RJSONIO) gives a bad result:

```
list(a=1, b=character(0))
```