To fix GH-2304 SaveSessionGatewayFilterFactory didn't work correctly

[imzhoukunqiang]

Hi Team, I'm upgrading from Zuul to Spring Gateway for my project. But I found a bug in SaveSessionGatewayFilterFactory .

Fixes GH-2304

public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
    return exchange.getSession().map(WebSession::save).then(chain.filter(exchange));
}

The return type of exchange.getSession().map(WebSession::save) is Mono<Mono<Void>>

I think this is a bug ,because the return value WebSession::save will never be consumed.

[pivotal-cla]

@imzhoukunqiang Please sign the Contributor License Agreement!

Click here to manually synchronize the status of this Pull Request.

See the FAQ for frequently asked questions.

[pivotal-cla]

@imzhoukunqiang Thank you for signing the Contributor License Agreement!