A path observer to an observable object should listen for any changes on that object

[jakemac53]

Repro: https://gist.github.com/jakemac53/aa2aceb4be88fdfd9efb

prints:

incrementing bar.foo.foo
Compound observer on `foo.foo` saw a change

when I would expect it to print:

incrementing bar.foo.foo
Compound observer on `foo` saw a change
Compound observer on `foo.foo` saw a change

[jakemac53]

I think this might actually be as designed, but if that is the case then I think the design is wrong

[jakemac53]

Ok, so I talked with @sigmundch about this and it is currently working as intended.

We should however provide some sort of interface for overriding this behavior so that if a path ends in an Observable it will also listen for changes on that object. This could probably be done with a named argument.