`respond_to? :split` is true for Redis::Namespace instance

https://github.com/sutajio/related/blob/master/lib/related.rb#L28

This test actually _passes_ when a Redis::Namespace object is passed... which is very strange, but is apparently expected Redis::Namespace behavior to retain interop with Redis for use within Resque.

See https://github.com/resque/redis-namespace/pull/18

Should probably simply check if this is a string, not just responds to a specific method. Any reason this was not the default test?