Modify comment_alter_taxonomy_admin_settings() to use array_filter

DamZ pointed out that FAPI has an undocumented feature that causes an array of checkboxes to be passed through array_filter() before saving using variable_set. An example of this usage is in http://api.drupal.org/api/function/book_admin_settings/6

I just committed a fix for this (http://drupal.org/cvs?commit=141515) despite what that wacky commit message states, but for the sake of good karma and smugness we should fix this once #311480: Refactor and simplify the comment form workflow is committed.