message: Change state field for authorization URL to have default
  This matches spotipy implementation
diff: @@ -350,6 +350,8 @@ impl SpotifyOAuth {
  payload.insert("scope", &self.scope);
  if let Some(state) = state {
  payload.insert("state", state);
  + } else {
  + payload.insert("state", &self.state);
  }
  if let Some(show_dialog) = show_dialog {
  if show_dialog {
  