Obey headers in template_preprocess_advanced_forum_topic_list_view() / _template_preprocess_advanced_forum_topic_list_view()

Currently, implementing template_preprocess_advanced_forum_topic_list_view() in a theme does not allow header information like the header_classes to actually be styled, because _template_preprocess_advanced_forum_topic_list_view() creates its own array of header information.

It would be nice if the variables passed-in were honored in that function.