msgpack::object's stream output format

[NorioKobota]

Hi, maintainers.

This is related to #273

std::map m = {{"key", msgpack::type::nil_t()}};
std::stringstream ss;
msgpack::pack(ss, m);
msgpack::object_handle oh = msgpack::unpack(ss.str().data(), ss.str().size());
std::cout << oh.get() << std::endl;

Above result is {"key":nil}.
I know the differences between nil and null, but unfortunately JSON does not have nil as reserved word.

So I'll be grad if use null instead of nil in operator<< function.

Regards.

[redboltz]

@NorioKobota , thank you for reporting the issue. I will replace nil with null for JSON output.

[redboltz]

merged.