docs: def header(self):
          """Generates HTTP header for this cookie."""
  
          if self._send_cookie:
              morsel = Morsel()
              cookie_value = Session.encode_sid(self._config.secret, self._sid)
              if self._config.encrypt_key:
                  cipher = AESCipher(self._config.encrypt_key)
                  cookie_value = cipher.encrypt(cookie_value)
  
                  if sys.version_info > (3, 0):
                      cookie_value = cookie_value.decode()
  
              morsel.set(self._config.cookie_name, cookie_value, cookie_value)
  
              # domain
              if self._config.domain:
                  morsel["domain"] = self._config.domain
  
              # path
              if self._config.path:
                  morsel["path"] = self._config.path
              # expires
              if self._expire_cookie:
                  morsel["expires"] = "Wed, 01-Jan-1970 00:00:00 GMT"
              elif self._backend_client.expires:
                  morsel["expires"] = self._backend_client.expires.strftime(EXPIRES_FORMAT)
  
              # secure
              if self._config.secure:
                  morsel["secure"] = True
  
              # http only
              if self._config.http_only:
                  morsel["httponly"] = True
  
              return morsel.OutputString()
          else:
              return None