The headers are not being passed to request

In our app, we are passing customized headers to `initReactivesearch` and not seeing the customized headers being passed to the actual request. I think the reason is that the `headers` always got set to an empty object `{}` in this line https://github.com/appbaseio/appbase-js/blob/develop/src/core/index.js#L58.

Shouldn't this line be `this.headers = config.headers || {}`? otherwise any AppBase instance would always have an empty object in `headers`.