Error parsing with IgnoreUnknownArguments to true and IEnumerable<int> option

Hi. I am having problems similar to described in #776 but working with version 2.9.1.

My scenario is this:

```csharp
using CommandLine;

namespace QuickStart
{
class Program
{
public class Options
{
[Option('i', "input", Required = true, HelpText = "The name of the videos to split. E.g. media split -i file1.mp4 file2.mp4")]
public IEnumerable Input { get; set; } = new List();

[Option('s', "seconds", Required = true, HelpText = "A space separated list of seconds to split the video. E.g. media split -s 5 10")]
public IEnumerable Seconds { get; set; } = new List();
}

static void Main(string[] args)
{
new Parser(with => { with.IgnoreUnknownArguments = true; })
.ParseArguments(args)
.WithParsed(o =>
{
Console.WriteLine("Quick Start Example!");
})
.WithNotParsed(errors =>
{
foreach (var e in errors)
{
Console.WriteLine($"Error: {e}");
}
});
}
}
}
```

Being IgnoreUnknownArguments = true when I pass more parameters (ex. `--input c:\temp\test.mp4 --seconds 2 4 --test1 xxxx`) it returns

```
Error: CommandLine.BadFormatConversionError
Error: CommandLine.MissingRequiredOptionError (in seconds option)
```

If I change the type of Seconds to `IEnumerable` it works correctly (this is going to be my workaround at this moment) but seems like library continues going crazy trying to parse unknown arguments in this scenario.

Start at Parser configuration and ParseArguments using the provided Options class, with IgnoreUnknownArguments enabled and IEnumerable for Seconds. Reproduce the command containing --test1 xxxx, then verify that the unknown arguments are ignored without producing BadFormatConversionError or MissingRequiredOptionError for seconds.

Written by the indexing model from the issue text.