Clarity on when deep path observers are called

[Michael Giuffrida]

When I observe a deep path of a property like so:

    observers: [

      'dataChanged(data.a.*')

    ]

Why is dataChanged called when data is set to {}? The change record is:

    { path: 'data.a',

      value: undefined,

      base: undefined }

Weirdly this only happens for the root property. If I observe dataChanged(data.a.b.*), then dataChange is called when data is set to {}, but not when data.a is set to {}.

What is the expected behavior here? Sample: http://jsbin.com/vibikewota/1/edit?html,console