Ignore property changes on non-configurables
Ignore property changes on non-configurables

In Safari, properties of hosted objects (particularly DOM elements) are
not configurable so observing a property on a DOM element, even if to
traverse to an observable object, would fail with an error. This change
allows observers to proceed on the assumption that the non-configurable
property will not be changed.

fredkilbourn/collections,cesine/collections,lu4/collections,cesarmarinhorj/collections,cesine/collections