jackson: support raw string as json response

[jknack]

Given:

{
  use(new Jackson());

  get("/raw", () -> {
     return "{\"json\": \"raw\"}";
  });
}

Produces a escaped json string and doesn't write the raw json string value. This is how Jackson works and there isn't much to do there.

Next release is going to allow to override this to support raw json strings:

{
  use(new Jackson().raw());

  get("/raw", () -> {
     return "{\"json\": \"raw\"}";
  });
}