# File lib/mixlib/authentication/http_authentication_request.rb, line 76
      def validate_headers!
        missing_headers = MANDATORY_HEADERS - headers.keys
        unless missing_headers.empty?
          missing_headers.map! { |h| h.to_s.upcase }
          raise MissingAuthenticationHeader, "missing required authentication header(s) '#{missing_headers.join("', '")}'"
        end
      end