code: function (pool, min, max) {
      var len
      switch (arguments.length) {
        case 0: // ()
          len = this.natural(3, 7)
          break
        case 1: // ( length )
          len = pool
          pool = undefined
          break
        case 2:
          // ( pool, length )
          if (typeof arguments[0] === 'string') {
            len = min
          } else {
            // ( min, max )
            len = this.natural(pool, min)
            pool = undefined
          }
          break
        case 3:
          len = this.natural(min, max)
          break
      }
  
      var text = ''
      for (var i = 0; i < len; i++) {
        text += this.character(pool)
      }
  
      return text
    }
docs: 返回一个随机字符串。