code: function(command, bodyLength, status) {
          if (null == bodyLength) {
              bodyLength = 0
          }
  
          if (null == status) {
              status = 0
          }
  
          // ----------- 存放1字节的command和1字节的status
          var buffer = new Buffer(2)
          buffer.writeUInt8(command, 0)
          buffer.writeUInt8(status, 1)
  
          // 生成8bytes存放body length的buffer
          var blBuffer = helpers.number2Buffer(bodyLength, 8)
  
          // 拼接
          return Buffer.concat([blBuffer, buffer])
      }
docs: 封装协议头
  @param command
  @param bodyLength
  @param status
  @return {Buffer}