joint.util.merge does not handle arrays properly

[DavidDurman]

joint.util.merge({ foo: [1, 2, 3] }, { foo: [4, 5] })

{
    "foo": [
        4,
        5,
        3
    ]
}

This causes an issue e.g. with joint.ui.SelectButton where if select-button-group is in inspector, arrays are merged instead of replaced.