json: Object properties should not be re-sorted alphabetically

Usually JSON documents have a meaningful ordering of properties, and rarely it is alphabetical.

With this library all JSON data gets sorted alphabetically, which is undesirable.

I guess it must be due to use of std::map.

Please describe the steps to reproduce the issue.

#include <iostream>
#include "json.hpp"

using Json = nlohmann::json;

int main() {
  std::cout << ">>> " << Json{{"C",1},{"B",2},{"A",3}}.dump() << std::endl;
}

What is the expected behavior?

>>> {"C":1, "B":2, "A":3}

And what is the actual behavior instead?

{“A”:3,“B”:2,“C”:1}

Which compiler and operating system are you using?

• Compiler: clang-7 and gcc-7
• Operating system: Ubuntu 18

Which version of the library did you use?

• [ x ] latest release version 3.7.3