rpc: Set HTTP Content-Type in bitcoin-cli
rpc: Set HTTP Content-Type in bitcoin-cli

We don't set any `Content-Type` in the client. It is more
consistent with our other JSON-RPC use to set it to `application/json`.

Note that our server doesn't enforce content types, so it doesn't make a
difference in practice. But it is fairly strange HTTP behavior to not set it.

This came up in #18950.


Former-commit-id: 88fdfa8d4746d56154decd5aa0ab6b920571b63c
syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin