def touch(key, ttl=nil)
      resp = perform(:touch, key, ttl_or_default(ttl))
      resp.nil? ? nil : true
    end
def touch(key, ttl=nil)
      resp = perform(:touch, key, ttl_or_default(ttl))
      resp.nil? ? nil : true
    end
Touch updates expiration time for a given key.

 Returns true if key exists, otherwise nil.