code: def get_fetch_headers(self, method, headers):
          
          all_headers = self.headers.copy()
          if headers:
              all_headers.update(headers)
          return Headers(all_headers)
docs: merge class headers with passed in headers
  
          :param method: string, (eg, GET or POST), this is passed in so you can customize
              headers based on the method that you are calling
          :param headers: dict, all the headers passed into the fetch method
          :returns: passed in headers merged with global class headers