Always return the result of .refresh in .retrieve

With the refactor of v13, there are now cases where `self` is not
mutated in the call to refresh and instead a new object is returned.
This change ensures that the new object is always returned by returning
the result of refresh instead.

lib/stripe/api_resource.rb

@@ -110,7 +110,6 @@ def self.retrieve(id, opts = {})
       opts = Util.normalize_opts(opts)
       instance = new(id, opts)
       instance.refresh
-      instance
     end
 
     def request_stripe_object(method:, path:, params:, base_address: :api, opts: {})