code: def buy_item(self, item_name, abbr):
          url = 'https://www.duolingo.com/2017-06-30/users/{}/purchase-store-item'
          url = url.format(self.user_data.id)
  
          data = {'name': item_name, 'learningLanguage': abbr}
          request = self._make_req(url, data)
  
          
  
          if request.status_code == 400 and request.json()['error'] == 'ALREADY_HAVE_STORE_ITEM':
              raise AlreadyHaveStoreItemException('Already equipped with ' + item_name + '.')
          if not request.ok:
              # any other error:
              raise Exception('Not possible to buy item.')
docs: status code '200' indicates that the item was purchased
          returns a text like: {"streak_freeze":"2017-01-10 02:39:59.594327"}