Set to true nullable bool arguments without the value

Consider the following class:

```csharp
class Parameters
{
[Option("bool")]
public bool Bool { get; set; }

[Option("nullable-bool")]
public bool? NullableBool { get; set; }
}
```

It works fine for such arguments: `--bool --nullable-bool true` but doesn't work for `--bool --nullable-bool`.

I suggest making it working for nullable bool types.

Reproduce the behavior with the Parameters class shown in the issue, comparing `--bool --nullable-bool true` with `--bool --nullable-bool`. Trace the command-line option parsing path for nullable bool properties; done means `--nullable-bool` sets NullableBool to true without requiring an explicit value, while existing boolean behavior remains intact.

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