I played around with this, I don't think we could make it work for instance doubles, since we only have the class handy. `method_missing` / `responds_to?` is instance dependent, so we would need to instantiate an object which isn't an option. It could be made to work for dynamically defined class methods, but I think it would be less confusing to just not support them at all.

I did find `method_defined?` though, which is a much nicer way of checking for method existence than the current approach.