Invokr supports Proc DuckTypes

The only place we weren't 100% certain is in builder line 34. We tried first to see if `method.respond_to? :call` would work but that caused other tests to fail. It seems though that if we reverse the order and check `respond_to? :name` things work.

Not sure if you have other suggestions, but this patch gives us what we need if you don't have any concerns.