Rapidjson: Pretty print to a string

[na-ka-na]

Feature request - pretty print a json to a string stream. Something like this:

std::ostringstream oss;
rapidjson::PrettyWriter(oss);
doc.Accept(oss);

[miloyip]

You can done it since #530

#include "rapidjson/ostreamwrapper.h"

std::ostringstream oss;
rapidjson::OStreamWrapper osw(oss);
rapidjson::PrettyWriter writer(osw);
doc.Accept(writer);