Support HTTPS.
HTTPS seams not supported.
```js
  let walletRpc = new MoneroWalletRpc(
    "https://x.x"
  )
  await walletRpc.openWallet("x", "x")
```
Error:
```bash
(node:12037) UnhandledPromiseRejectionWarning: Error: RequestError: TypeError [ERR_INVALID_PROTOCOL]: Protocol "https:" not supported. Expected "http:" at MoneroRpcConnection.sendJsonRequest (/home/user/Projects/jointed.gitlab.io/docker/node_modules/monero-javascript/src/main/js/rpc/MoneroRpcConnection.js:124:18)
```

It is possible to connect to monero-wallet-rpc over https with curl.
```bash
curl -X POST https://x.x/json_rpc -d '{"jsonrpc":"2.0","id":"0","method":"open_wallet","params":{"filename":"x","password":"x"}}' -H 'Content-Type: application/json'
{
  "id": "0",
  "jsonrpc": "2.0",
  "result": {
  }
}
```