CollectionProxy#replace should change the DB records rather than just mutating...

CollectionProxy#replace should change the DB records rather than just mutating the array. Fixes #3020.

activerecord/lib/active_record/associations/collection_proxy.rb

@@ -46,7 +46,7 @@ class CollectionProxy # :nodoc:
 
       delegate :select, :find, :first, :last,
                :build, :create, :create!,
-               :concat, :delete_all, :destroy_all, :delete, :destroy, :uniq,
+               :concat, :replace, :delete_all, :destroy_all, :delete, :destroy, :uniq,
                :sum, :count, :size, :length, :empty?,
                :any?, :many?, :include?,
                :to => :@association

activerecord/test/cases/associations/has_many_associations_test.rb

@@ -1578,4 +1578,15 @@ def test_association_attributes_are_available_to_after_initialize
 
     assert_equal car.id, bulb.attributes_after_initialize['car_id']
   end
+
+  def test_replace
+    car = Car.create(:name => 'honda')
+    bulb1 = car.bulbs.create
+    bulb2 = Bulb.create
+
+    assert_equal [bulb1], car.bulbs
+    car.bulbs.replace([bulb2])
+    assert_equal [bulb2], car.bulbs
+    assert_equal [bulb2], car.reload.bulbs
+  end
 end