added test for available_for_each in article
added test for available_for_any(provider) in article
corrected available_for_each(provider) test
added test for is_book? in article
added the get_price test for article
added test for to_s
added add_comment test for article
Merge branch 'develop' into feature/article_test
added test for average_rating in article
added test for get_price_of_day
added test for generate in article
Merge branch 'feature/article_test' into develop