def expected_value(rules, multi_keys)
      multi_keys.inject(rules) { |o, key| o[key] }
    end
def expected_value(rules, multi_keys)
      multi_keys.inject(rules) { |o, key| o[key] }
    end
Get expected value from rules.

 @param rules [Hash]
 @param multi_keys [Array<Symbol>]
 @return [Object] expected value.