First segment in SegmentGroup not included (issue 149 revisited)

Hello,

When I change the EdiFact_Issue149_SegmentGroups_SG26 class in issue 149 to the below (Using a property of type EdiFact_Issue149_PAI instead of inheriting from the class) i fails to load the PAI property.

Should this not be possible?

```csharp
[EdiSegmentGroup("PAI", "RFF", "MOA")]
public class EdiFact_Issue149_SegmentGroups_SG26
{
**public EdiFact_Issue149_PAI PAI { get; set; }**
public EdiFact_Issue149_MOA MOA { get; set; }
public List References { get; set; }
}
```

Start with the EdiFact_Issue149_SegmentGroups_SG26 example and the EdiSegmentGroup("PAI", "RFF", "MOA") attribute shown in the issue. Reproduce deserialization with PAI as a property rather than a base class, then verify that the PAI property is populated and the first segment is included in the SegmentGroup.

Written by the indexing model from the issue text.