Add tests for Question::Date#to_response
Add tests for Question::Date#to_response

I'm looking to modify the behaviour of this method but wanted some tests in
place first to give me some confidence.

stwalsh/smart-answers,aledelcueto/smart-answers,alphagov/smart-answers,stwalsh/smart-answers,alphagov/smart-answers,aledelcueto/smart-answers,aledelcueto/smart-answers,alphagov/smart-answers,aledelcueto/smart-answers,stwalsh/smart-answers,stwalsh/smart-answers,alphagov/smart-answers