NPE when building a child context with combined map

[bckfnn]

Using 0.2.1. snaphot, I get a NPE when I create child2 since the extendedContext of a child is Null.

    Context root = Context.newBuilder("root").build();
    System.out.println(root.get("this"));
    Context child1 = Context.newBuilder(root, "child1").build();
    System.out.println(child1.get("this"));
    Context child2 = Context.newBuilder(root, "child2").combine(new HashMap<String, Object>()).build();

The use case is a #each like iterator that also add odd/even flags to the namespace.

[jknack]

fixed