fix respond_to test for protected method

`should respond_to` doesn't allow you to pass in the `include_all` boolean parameter to check against protected/private methods. (See: http://ruby-doc.org/core-2.1.1/Object.html#method-i-respond_to-3F)

Therefore we need to check against the native `respond_to?` method instead.

@joshuafleck @dawid-sklodowski please review/sign-off