code: function(metaData) {
          var first = true
          var result = ''
  
          Object.keys(metaData).forEach(function(key) {
              if (!first) {
                  result += protocol.FDFS_RECORD_SEPERATOR
  
              } else {
                  first = false
              }
              var value = metaData[key]
              result += key
              result += protocol.FDFS_FIELD_SEPERATOR
              result += value
          })
  
          return result
      }
docs: 封装meta data
  注：返回的是string，由外部写入到buffer中
  @param {Object} metaData
  @return {String}