Fix CollectionAssociation#replace to return new target (closes rails#6231)

Conflicts:

	activerecord/test/cases/associations/has_many_associations_test.rb

Author: drogus

activerecord/lib/active_record/associations/collection_association.rb

@@ -481,6 +481,8 @@ def replace_records(new_target, original_target)
             raise RecordNotSaved, "Failed to replace #{reflection.name} because one or more of the " \
                                   "new records could not be saved."
           end
+
+          new_target
         end
 
         def concat_records(records)

activerecord/test/cases/associations/has_many_associations_test.rb

@@ -1686,4 +1686,16 @@ def test_replace
     assert_equal [bulb2], car.bulbs
     assert_equal [bulb2], car.reload.bulbs
   end
+
+  def test_replace_returns_new_target
+    car = Car.create(:name => 'honda')
+    bulb1 = car.bulbs.create
+    bulb2 = car.bulbs.create
+    bulb3 = Bulb.create
+
+    assert_equal [bulb1, bulb2], car.bulbs
+    result = car.bulbs.replace([bulb1, bulb3])
+    assert_equal [bulb1, bulb3], car.bulbs
+    assert_equal [bulb1, bulb3], result
+  end
 end