Add failing test for guid list format.

Hi Demis,

We have noticed that if we have an entity that has an enumerable of Guids as one of it's properties, it stores the values in the 32 digit (N) format.  This isn't a direct problem, in terms of serialising and deserialising, but it is a bit of an issue if through Sql Server Studio Manager you want to copy the value out and check it against another source table, where the values are true Guids.

I've traced the actual cause down to a function in ServiceStack.Text, 

```
public void WriteGuid(TextWriter writer, object oValue)
{
writer.Write(((Guid)oValue).ToString("N"));
}
```

Ideally, we'd be able to control this serialisation in some way.  Any thoughts?

Thanks

Richard