respond_to? by default (in ruby) requires 2 args

That fixes rspec 3 support and makes everything a little bit more
consistent:
rails/rails@ba5fab4

lib/choices/rails.rb

@@ -25,7 +25,7 @@ def from_file(name)
     end
   end
 
-  def respond_to?(method)
+  def respond_to?(method, include_private = false)
     super or method.to_s =~ /=$/ or (method.to_s =~ /\?$/ and @choices.key?($`))
   end