def on_chunk(&block)
      raise "Body already read" if @on_chunk
      raise ArgumentError, "block must be given" unless block_given?
      @on_chunk = block
      unless @body.empty?
        @body.each(&block)
        @body.clear
      end
      self
    end
def on_chunk(&block)
      raise "Body already read" if @on_chunk
      raise ArgumentError, "block must be given" unless block_given?
      @on_chunk = block
      unless @body.empty?
        @body.each(&block)
        @body.clear
      end
      self
    end
Set callback that will be called when received a chunk of response body.
 @yield [chunk] A chunk of the response body.