damn that oAuth crap!
+1 on that. I was also getting this exception when migrating on older Ruby 1.8.7/Rails 2 app over to OAuth. Ended up getting into Gem dependency hell trying to upgrade the Shopify API gem thinking that was the problem and eventually gave up and rewrote the solution using another approach (using Shopify App gem) but I'm pretty sure it was simply this problem all along.
you'd expect a 403
Indeed.