Prevent reading unknown attributes on a resource

At present resources accept calls to any method and return nil. This behaviour may lead to hard to track bugs. My proposition is to raise an error in such case. Of course setting previously unknown attribute should be perfectly possible.

I have run specs for some services with this patch and they passed. Unfortunately some services use old versions of api_client / fs-api and update was not straightforward thus I could not test it easily. I think however that this change can only help exposing existing bugs which is a good thing. One downside is that poorly mocked specs may fail because of it.