This update topic route has never worked. Better late than never. I am
in favor of using non-slug urls when using the api so I do think we
should fix this route.
Just thought I would update the `:id` param to `:topic_id` here in the
routes file instead of updating the controller to handle both params.
Added a spec to test this route.
Also added the same constraint we have on other topic routes to ensure
we only pass in an ID that is a digit.