Adding implementation of `respond_to?` for easier mocking/better API

Hello.

I think it's customary when implementing `method_missing` to also implement `respond_to?`, so that the method logic that's happening in `method_missing` is reflected in the public API of the class.

This, for example, makes mocking with RSpec must easier, since rspec-mocks checks the object to be sure it implements a method you're stubbing before you're allowed to stub it.

Thanks!