Aaah thanks to @mortenrøgenes- turns out that I have to wrap the Addresses property of the Neighbourhood class into it's own type as such:
public class Addresses
{
public Street Street{ get; set; }
public HashSet<int> HouseNumbers { get; set; }
}
public class Neighbourhood
{
public Addresses { get; set; }
public string LocalCouncilName { get; set; }
public string RegionName { get; set; }
}
That way the ServiceStack JSONSerializer would work and the test would pass.