adds a failing test for 12475

This adds a failing test for #12475. It appears that `obj.set('array', [a1, a2, a3])` fails to install contentKey observers on the new array value so that the subsequent changes to `foo` are ignored.

packages/ember-runtime/tests/computed/reduce_computed_macros_test.js

@@ -6,6 +6,7 @@ import { get } from 'ember-metal/property_get';
 import { set } from 'ember-metal/property_set';
 import { addObserver } from 'ember-metal/observer';
 import { observer } from 'ember-metal/mixin';
+import { computed } from 'ember-metal/computed';
 import {
   sum,
   min,
@@ -427,6 +428,28 @@ QUnit.test('properties values can be replaced', function() {
   deepEqual(obj.get('a1bs').mapBy('name'), ['item1'], 'properties can be filtered by matching value');
 });
 
+QUnit.test('responds to change of property value on element after replacing array', function() {
+  let a1 = { foo: true, id: 1 };
+  let a2 = { foo: true, id: 2 };
+  let a3 = { foo: true, id: 4 };
+
+  obj = EmberObject.extend({
+    a: computed('[email protected]', function() {
+      return this.get('array').filter(elt => elt.foo).reduce(((a,b) => a + b.id), 0);
+    })
+  }).create({
+    array: emberA([a1, a2])
+  });
+
+  deepEqual(obj.get('a'), 3, 'value is correct initially');
+  set(a1, 'foo', false);
+  deepEqual(obj.get('a'), 2, 'responds to change of property on element');
+  obj.set('array', [a1, a2, a3]);
+  deepEqual(obj.get('a'), 6, 'responds to content array change');
+  set(a1, 'foo', true);
+  deepEqual(obj.get('a'), 7, 'still responds to change of property on element');
+});
+
 [
   ['uniq', uniq],
   ['union', union]