Fixed #respond_to? on ActiveRecord::NamedScope::Scope.

The Scope class delegates most of the standard methods to the @proxy_found array. It also does this for #respond_to? which breaks stuff, for instance working with test/spec which properly checks if a method exists before dispatching.

This patch makes #respond_to? on instances of Scope work again, but it will also check if the @proxy_scope responds to the message, thus going up the chain to ActiveRecord::Base.

It also changes [].methods to Array.instance_methods, because that's just a bit cleaner.