Fix explicit serializer for associations

[wjordan]

I needed to apply this patch in order to get the explicit association serializers feature to work as expected with the has_many association. Looked like a probable typo (the feature PR #696 renamed :options to :association_options, and forgot to update this one to match).

I've updated the test_includes_linked_comments test with the proper expected result (the old test was not invoking the explicit has_many association serializer functionality at all).