Guard against invalid date in params[:response]
Guard against invalid date in params[:response]

An error can occur if `params[:response]` contains invalid data.

Fixes the following error:

Error:
SmartAnswersControllerDateQuestionTest#test_: GET /<slug> date question should not error if passed string response. :
ActionView::Template::Error: undefined method `dig' for "bob":String
    app/presenters/date_question_presenter.rb:18:in `selected_day'
    app/views/smart_answers/inputs/_date_question.html.erb:17
    app/views/smart_answers/inputs/_date_question.html.erb:1
    app/views/smart_answers/question.html.erb:40
    app/views/smart_answers/question.html.erb:18
    app/controllers/smart_answers_controller.rb:29:in `show'
    test/functional/smart_answers_controller_test_helper.rb:9:in `submit_response'
    test/functional/smart_answers_controller_date_question_test.rb:66:in `submit_response'
    test/functional/smart_answers_controller_date_question_test.rb:41:in `block (3 levels) in <class:SmartAnswersControllerDateQuestionTest>'
    test/functional/smart_answers_controller_date_question_test.rb:48:in `instance_exec'
    test/functional/smart_answers_controller_date_question_test.rb:48:in `block in create_test_from_should_hash'

rails test test/functional/smart_answers_controller_date_question_test.rb:40

alphagov/smart-answers,alphagov/smart-answers,alphagov/smart-answers,alphagov/smart-answers