message: Don't unwrap nkey Option
diff: @@ -201,11 +201,13 @@ impl Server {
  let info = crate::parser::expect_info(&mut stream)?;
  if !info.nonce.is_empty() {
  - let nkey = nkey.unwrap();
  - let sig = nkey.sign(info.nonce.as_bytes()).unwrap();
  + if let Some(nkey) = nkey {
  + let sig = nkey.sign(info.nonce.as_bytes())
  + .map_err(|err| Error::new(ErrorKind::Other, err))?;
  let sig = base64_url::encode(&sig);
  connect_op.sig = Some(sig);
  }
  + }
  let op = format!(
  "CONNECT {}\r\nPING\r\n",
  