def stack(options={}, &block)
      @stack ||= begin
        if block_given?
          Faraday::RackBuilder.new(&block)
        else
          Faraday::RackBuilder.new(&default_middleware(options))
        end
      end
    end
def stack(options={}, &block)
      @stack ||= begin
        if block_given?
          Faraday::RackBuilder.new(&block)
        else
          Faraday::RackBuilder.new(&default_middleware(options))
        end
      end
    end
Exposes middleware builder to facilitate custom stacks and easy
 addition of new extensions such as cache adapter.