InvalidCastException when parsing nullable enum

Hi there,

my code looks like this:

```
// Code in Main
using (var parser = Parser.Default)
{
var parserResult = parser.ParseArguments(args);
}

// CommandlineOptions Class
public class CommandlineOptions
{
[Option('c', "command", Required = true, HelpText = "The Command to use")]
public ECommand? Command { get; set; }
}

// Enum ECommand
public enum ECommand
{
DoStuff = 1;
DoSomethingElse = 2;
}
```

Notice that my Command-Option is a nullable enum.

When i run my programm with the following command
`myProgramm.exe -c DoStuff`
I get an `InvalidCastException`, saying that it can't convert `System.String` in `ECommand`.
This exception is thrown by` parser.ParseArguments()`.

Am I missing something or is this a bug?
How can i fix that?

I know that the exception doesn't occure if the enum isn't nullable. But I need the nullable enum at that point!

Hope you can help me.
Greetings Tom

首先，使用 nullable ECommand? 屬性和 `-c DoStuff` 引數，透過 parser.ParseArguments() 重現該故障。追蹤 nullable enum 的轉換路徑；當命令能夠在沒有 InvalidCastException 的情況下完成剖析並指派預期的 enum 值時，即表示完成。

由索引模型根據 Issue 內容生成。