code: def request_headers
        headers = { 'X-Contentful-User-Agent' => contentful_user_agent }
        headers['Authorization'] = "Bearer #{configuration[:access_token]}" if configuration[:authentication_mechanism] == :header
        headers['Content-Type'] = "application/vnd.contentful.delivery.v#{configuration[:api_version].to_i}+json" if configuration[:api_version]
        headers['Accept-Encoding'] = 'gzip' if configuration[:gzip_encoded]
        headers
      end
docs: Returns the headers used for the HTTP requests
   @private