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

まず、nullable ECommand? プロパティと `-c DoStuff` 引数を使って parser.ParseArguments() 経由で失敗を再現します。nullable enum の変換経路を追跡します。コマンドが InvalidCastException なしで解析され、期待される enum 値が割り当てられれば完了です。

索引モデルが issue の本文から書いたものです。