val message6 = message2 + message2
    client.send(message6)
    serverListener.assertTextMessage(message6)
    closeWebSockets(client, server)
    val upgradeRequest = webServer.takeRequest()
    assertThat(upgradeRequest.headers["Sec-WebSocket-Extensions"])
      .isEqualTo("permessage-deflate")
  }

  @Throws(Exception::class)
  private fun extensionNegotiationFailure(extensionsHeader: String) {