isDestroying is false when x-option (& x-select) are being destroyed (Ember 2.8 beta)

[Robdel12]

When you navigate to the dummy apps /ember-data route and then try to navigate away look in the console for an error. Should be:

Uncaught Error: Attempted to handle event `didSetProperty` on <dummy@model:person::ember555:null> while in state root.deleted.saved. Called with {name: name, oldValue: undefined, originalValue: undefined, value: null}.

This is because we're trying to set the value on a deleted ember data object. What's happening is here this.get('isDestroying') is false which means we're going to call _updateValue() and go 💥 .

Possibly related to:
emberjs/ember.js#13775
emberjs/ember.js#14110
emberjs/ember.js#14159

Version of x-select:

v2.2.1

Ember Version / Ember CLI Version:

Ember Version: Ember beta
Ember CLI Version: 2.7

Expected Behavior

Navigate to the /ember-data route in the dummy app and then navigate away. There should be no console errors.

Actual Behavior

When you navigate away from the /ember-data route it goes 💥

Steps To Reproduce:

1. Install ember 2.8 beta.
2. Navigate to /ember-data
3. Navigate away from that route.
4. Look at the console

[Robdel12]

Closed with #142