springcloud gateway转发websocket时报类型转换异常

使用spring-boot-starter-websocket搭建了websocket服务，直接用服务的IP加端口访问是正常的，换成网关的访问地址就报错。

查看日志，

2020-12-17 18:26:43,675 ERROR AbstractErrorWebExceptionHandler:117 -[d5c6b807] 500 Server Error for HTTP GET "/websocket/pk/ssss/1"
java.lang.ClassCastException: com.spinning.gateway.filters.PostGatewayFilter$1 cannot be cast to org.springframework.http.server.reactive.AbstractServerHttpResponse
	at org.springframework.web.reactive.socket.server.upgrade.ReactorNettyRequestUpgradeStrategy.upgrade(ReactorNettyRequestUpgradeStrategy.java:75) ~[spring-webflux-5.1.5.RELEASE.jar:5.1.5.RELEASE]

刚开始怀疑是网关的配置问题，

- id: websocket服务接口
  uri: lb:ws://websocket
  predicates:
    - Path=/websocket/**
  filters:
    - StripPrefix=1

根据官网检查了springcloud gateway的配置，也没发现问题。看这段日志，有一个自己写的类，应该是这里的问题。

@Override
    public Mono<Void> filter(ServerWebExchange exchang