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.
Merge pull request #9870 from senny/9806_eager_loading_regression

do not reset associations when preloading twice.