do not reset associations when preloading twice.

Closes #9806.

As the `through_options` always contained `{:order=>nil}` the second time,
the preloader ran, the association was always reset. This patch only
adds the `:order` to the `through_options` if it is set.

activerecord/test/cases/associations/eager_test.rb

@@ -1118,4 +1118,11 @@ class EagerAssociationTest < ActiveRecord::TestCase
     assert_no_queries { assert_equal 2, author.comments_with_order_and_conditions.size }
     assert_no_queries { assert_equal 5, author.posts.size, "should not cache a subset of the association" }
   end
+
+  test "preloading a through association twice does not reset it" do
+    members = Member.includes(current_membership: :club).includes(:club).to_a
+    assert_no_queries {
+      assert_equal 3, members.map(&:current_membership).map(&:club).size
+    }
+  end
 end