Revalidates arrays when they are replaced.

When an array was replaced the validator was not being updated, it was just validating the original array.

This problem occurred especially in computed properties that depend on arrays.

```javascript
model = Ember.Object.extend({
   items: [],
   visibleItems: function() {
     // a new array will be created every time this computed property is updated,
     // but just the original array was being validated.
     return this.get('myArray').filterBy('isDeleted', false);
  }.property('items.@each.isDeleted')
});

model.reopen({
  validations: {
    visibleItems: true
  }
});
```