Device a way to retrieve response headers

The solution as follows:

1) We save the response headers inside `CoreGsonRequest`.
2) We keep the `CoreGsonRequest` created on the fly in `CoreBaseRequest` as a member.
3) We implement an accessor method in `CoreBaseRequest` to be able to get response headers from `CoreGsonRequest`.

This way is CoreGsonRequest is not directly visible to individual requests in application code, and response headers are available as `this.getResponseHeaders()` inside `onSuccess()` listener.