Add Content-Type: application/json to rest client

Tried to use bitcoin-client with https://blockchain.info/api/json_rpc_api and it kept failing. Turns out the reason is that RestClient uses application/x-www-form-urlencoded by default, which is wrong as we are sending application/json. Apparently blockchain.info is strict about proper content-type.

bitcoind does not seem to care about content-type, so it wouldn't hurt it.