Question Spec 추가

### 1. Question 사용하지 않는 FactoryGirl 제거 ###

- shoulda-matcher를 활용하였기에, answer를 추가하는 Question FactoryGirl 부분은 삭제함.

**spec/factories/questions.rb**

    factory :question_with_answers do
      ignore do
        answers_count 1
      end
    
      after(:create) do |question, evaluator|
        create_list(:answer, evaluator.answers_count, question: question)
      end
    end

### 2. Question : plaza 관계 추가 ###

- (post 모델과 동일하게) question 모델에 after_create filter로 plaza 생성.

**app/models/question.rb**

    after_create :set_plaza_question
    private
      def set_plaza_question
        self.create_plaza
      end

- question 모델 spec에 spec 추가

**spec/models/question_spec.rb**

    it "> has_one :plaza, dependent: :destroy" do
      expect(create(:question)).to have_one(:plaza).dependent(:destroy)
    end