AstroBWT algorithm (DERO) support

To test:

- Download https://github.com/deroproject/derosuite/releases/tag/AstroBWT
- Run daemon with `--testnet` in command line

In config.json:
- "coin":"dero"
- "url":"127.0.0.1:30306"
- "daemon:"true"

src/base/net/http/HttpClient.cpp

@@ -140,6 +140,7 @@ void xmrig::HttpClient::handshake()
 
     if (!body.empty()) {
         headers.insert({ "Content-Length", std::to_string(body.size()) });
+        headers.insert({ "Content-Type", "application/json" });
     }
 
     std::stringstream ss;