rbuf_consume

# File lib/net/protocol.rb, line 229
    def rbuf_consume(len)
      if len == @rbuf.size
        s = @rbuf
        @rbuf = ''.b
      else
        s = @rbuf.slice!(0, len)
      end
      @debug_output << %[-> #{s.dump}\n] if @debug_output
      s
    end