Json post bug

[ce2020-ai]

I built a request using nlohmann:: JSON and wrote the code according to the example, but I encountered the following bug:

terminate called after throwing an instance of 'std::length_error'
what(): basic_string::_S_create
Aborted

You should use a JSON library like https://github.com/nlohmann/json for JSON encoding. JSON is not a part of the HTTP standard (RFC 7231) so it is out of the scope of this library. I added an example on hot wo send a JSON POST in b6b8795.

Originally posted by @elnormous in #38 (comment)