Add Fulfillment#cancel

lib/shopify_api/resources/fulfillment.rb

@@ -1,5 +1,7 @@
 module ShopifyAPI
   class Fulfillment < Base
     self.prefix = "/admin/orders/:order_id/"
+
+    def cancel; load_attributes_from_response(post(:cancel, {}, only_id)); end
   end
 end