4.0.0-beta.1 版本数组模型转换 toJSON 的时候会有 Optional 关键字？

模型：
```swift
struct WNResume: HandyJSON {
// 这里有其他属性，先不管
/// 技能培训
struct SkillTraining: HandyJSON {
var star_time: UInt64? // "时间戳",
var end_time: UInt64? // "时间戳",
var skill: String? // "技能",
var skill_content: String? // "技能描述"
}
}
```

我往SkillTraining 数组添加数据之后，并将其 toJSON 后作为参数传递给后台：
```swift
var skillTrainings = [WNResume.SkillTraining]()
for i in 0 ..< 5 {
var skill = WNResume.SkillTraining()
skill.end_time = 12
skill.star_time = 34
skill.skill = "12 \(i)"
skill.skill_content = "666 \(i)"
skillTrainings.append(skill)
}

print(skillTrainings.toJSON())
```
打印结果如下：

我想要去掉 Optional 关键字，我尝试过将所有属性置为不可选，即类似：
```swift
var end_time: UInt64 = 0 // "时间戳",
var skill: String = "" // "技能",
```

依旧不起作用。

我看到在 https://github.com/alibaba/HandyJSON/issues/160 说在 1.8.0 版本已经修复了这个，但我在4.0.0-beta.1 版本下还是有这个问题，望解答。

Reproduce the issue with the WNResume.SkillTraining model and its array's toJSON call, comparing optional and non-optional properties. Trace the serialization path used by toJSON; the work is done when the generated JSON no longer contains Optional wrappers for these values and the behavior is covered by a regression check.

Written by the indexing model from the issue text.