OverridingContainer should extend base enumerables

[LordZoltan]

The Auto-built enumerables should concatenate any objects from an overriding container with those of the base container, so:

var container1 = new Container();
container1.RegisterObject(1);
var container2 = new OverridingContainer(container1);
container2.RegisterObject(2);

var result = container2.Resolve<IEnumerable<int>>();

// result should be [1, 2]