Use corerct way to respond to missing

Hi,

The number of the arguments of `respond_to?` is 2, `name` and `include_private = false`.
http://ruby-doc.org/core-2.1.2/Object.html#method-i-respond_to-3F
http://www.ruby-doc.org/core-1.9.3/Object.html#method-i-respond_to-3F
If you use `respond_to?(:foo, true)` on these classes, which I fixed on, it raises `ArgumentError`. 

Furthermore, we should use `respond_to_missing?` instead of `respond_to?` if we want to override the behavior.
http://robots.thoughtbot.com/always-define-respond-to-missing-when-overriding