Make nesting work for newly added child observables

[egasimus]

Suppose you create an ObservStruct and then decide to add more observable keys to it at some later point. Well, since all the nice magic only happens on initialization, that doesn't really work. For example, if you do something like:

foo = Observ("foo")
bar = Observ("bar")
baz = ObservStruct({ foo: foo })
baz.set(extend(baz(), { bar: bar }))

then baz() would return { foo: "foo", bar: [Function: observable] } rather than { foo: "foo", bar: "bar" }; and baz.bar would also remain undefined rather than becoming a proper reference to bar.

That behavior was getting in my way -- I'm putting together a simple observable key-value store for individually observable models, to serve as a state container for my app which involves a lot of user-created instances of said models -- so I had a stab at fixing it. Let me know what you think?

[Raynos]

@egasimus

Have you considered using https://github.com/nrw/observ-varhash ?