def _view_url(preference)
      uri = URI(request.url)
      query = Rack::Utils.parse_nested_query(uri.query)
      query["prefer"] = preference
      uri.query = query.to_param
      uri.to_s
    end
def _view_url(preference)
      uri = URI(request.url)
      query = Rack::Utils.parse_nested_query(uri.query)
      query["prefer"] = preference
      uri.query = query.to_param
      uri.to_s
    end
preference: m, f (mobile, full)