IronMultiSelectableBehavior: no _updateSelected() on DOM mutation

[sklobovskaya]

IronMultiSelectableBehavior fails to update the selection (this._updateSelected()) when the DOM is mutated.

IronSelectableBehavior calls this._updateSelected() on DOM mutation, here:

https://github.com/PolymerElements/iron-selector/blob/master/iron-selectable.html#L295

However, when using IronMultiSelectableBehavior, the above line is never executed because IronMultiSelectableBehavior uses this.selectedValues instead, and thus this.selected is always null.

[eliseosoto]

The problem appears to be in IronSelectableBehavior._observeContent

https://github.com/PolymerElements/iron-selector/blob/master/iron-selectable.html#L277

var content = node.querySelector('content');
content is null

And then after an element that has IronSelectableBehavior is attached the _contentObserver is undefined.