(Http-Client) Support Jsonb SubTypes in request body

[SentryMan]

The use of .getClass() means that jsonb subtypes cannot work.

@Client
public interface Client {

  @Post("/resource")
  public Response createResource(ExtendableObject object);

}

Providing an ExtendingObject that extends ExtendableObject means that the JsonAdapter for ExtendableObject is not used, so the @type field is lost. This PR fixes that.

• Adds new body methods to HttpClientRequest to take the type parameter
• now generated clients will provide the type parameters