Add complete action for Fulfillment endpoint

lib/shopify_api/resources/fulfillment.rb

@@ -3,5 +3,6 @@ module ShopifyAPI
     self.prefix = "/admin/orders/:order_id/"
 
     def cancel; load_attributes_from_response(post(:cancel, {}, only_id)); end
+    def complete; load_attributes_from_response(post(:complete, {}, only_id)); end
   end
 end