Seperator does not appear in help text when diplaying default values

I have the following property in the options:

```
[Option("branch", Required = false, Separator = ',', HelpText = "Branches to include.",
Default = new[] { "master", "breakfix" })]
public IEnumerable IncludeBranches { get; set; }
```

The separator is set to ',' then a list of string is given for the default.

The help output is:

```
--branch (Default: master breakfix) Branches to include.
```

This would be miss-leading to the user since it does not include the comma. Would it instead make sense to do something like this?

```
--branch (Default: master,breakfix) Branches to include.
```

从 issue 的 Option 配置复现帮助输出，重点关注当 Separator 设置为逗号时，默认 IEnumerable 值是如何呈现的。跟踪用于格式化默认值的帮助生成入口点，并验证完成的输出表示了已配置的分隔符，同时不更改无关的帮助文本。

由索引模型根据 Issue 内容生成。