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

@@ -427,6 +427,26 @@ 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 };
+  let a2 = { foo: true };
+  let a3 = { foo: true };
+
+  obj = EmberObject.extend({
+    a: filterBy('array', 'foo')
+  }).create({
+    array: emberA([a1, a2])
+  });
+
+  deepEqual(obj.get('a'), [a1, a2], 'value is correct initially');
+  set(a1, 'foo', false);
+  deepEqual(obj.get('a'), [a2], 'responds to change of property on element');
+  obj.set('array', [a1, a2, a3]);
+  deepEqual(obj.get('a'), [a2, a3], 'responds to content array change');
+  set(a1, 'foo', true);
+  deepEqual(obj.get('a'), [a1, a2, a3], 'still responds to change of property on element');
+});
+
 [
   ['uniq', uniq],
   ['union', union]