def write(str)
      s = str.to_s
      @length += Rack::Utils.bytesize(s)
      @writer.call s

      header["Content-Length"] = @length.to_s
      str
    end
def write(str)
      s = str.to_s
      @length += Rack::Utils.bytesize(s)
      @writer.call s

      header["Content-Length"] = @length.to_s
      str
    end
Append to body and update Content-Length.

 NOTE: Do not mix #write and direct #body access!