fix `clients.foos` syntax from using classes outside of the appropriate namespace

You can fetch the `Zendesk::Voice::Greeting` resource using `client.greetings`. Previous to this fix, if you also had a top-level `::Greeting` class defined, it would attempt to use that class instead, raising an exception. This happens because `Module#const_defined?` searches ancestors by default.

This also re-applies #185.