Stop RJSONIO from simplifying content by default.

I noticed tests were failing when `content$*` was a vector instead of a list; it looks like RJSONIO simplifies by default. I've wired in a flag that will override this unless the user has requested otherwise.

It occurs to me now that I grepped for `fromJSON`, but not `toJSON`. The latter seems safe enough? Let me know if you want me to dig more.