SlugColumnHandler "eats" input on Help import #63584934

Per [bug 63584934](https://www.pivotaltracker.com/story/show/63584934); previous attempts at using the designated `SlugColumnHandler` -- this time with the slug being required -- did not work; this PR makes it work correctly.  Slug value is required, and setting to an existing slug's value now correctly makes it update that item.