code: func (v *StringLengthInRange) IsValid(errors *validate.Errors) {
  	strLength := utf8.RuneCountInString(v.Field)
  	if v.Max == 0 {
  		v.Max = strLength
  	}
  	if v.Message == "" {
  		v.Message = fmt.Sprintf("%s not in range(%d, %d)", v.Name, v.Min, v.Max)
  	}
  	if !(strLength >= v.Min && strLength <= v.Max) {
  		errors.Add(GenerateKey(v.Name), v.Message)
  	}
  }
docs: // IsValid checks that string in range of min:max
  // if max not present or it equal to 0 it will be equal to string length