Add missing spec coverage for related resources operations (#144)

I discovered that this simple scenario wasn't tested properly when
working on JR 0.10 support

Author: valscion

spec/requests/related_resources_operations_spec.rb

@@ -66,6 +66,7 @@
 
     context 'authorized for show_related_resource' do
       before { allow_operation('show_related_resource', source_record: article, related_record: article.author) }
+      it { is_expected.to be_ok }
 
       # If this happens in real life, it's mostly a bug. We want to document the
       # behaviour in that case anyway, as it might be surprising.