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.
```

Reproduce the help output from the issue's Option configuration, focusing on how the default IEnumerable values are rendered when Separator is set to a comma. Trace the help-generation entry point that formats default values and verify that the completed output represents the configured separator without changing unrelated help text.

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