Fix Kotlin filter parameter bug in Router DSLs

[ShindongLee]

spring-framework/spring-webmvc/src/main/kotlin/org/springframework/web/servlet/function/RouterFunctionDsl.kt

Lines 650 to 655 in 5b1ab31

	fun filter(filterFunction: (ServerRequest, (ServerRequest) -> ServerResponse) -> ServerResponse) {
	builder.filter { request, next ->
	filterFunction(request) {
	next.handle(request)
	}
	}

spring-framework/spring-webflux/src/main/kotlin/org/springframework/web/reactive/function/server/CoRouterFunctionDsl.kt

Lines 531 to 539 in 5b1ab31

	fun filter(filterFunction: suspend (ServerRequest, suspend (ServerRequest) -> ServerResponse) -> ServerResponse) {
	builder.filter { serverRequest, handlerFunction ->
	mono(Dispatchers.Unconfined) {
	filterFunction(serverRequest) {
	handlerFunction.handle(serverRequest).awaitSingle()
	}
	}
	}
	}

1. Function filterFunction takes serverRequest and function handler as parameters. (I just name it second parameter of filterFunction as handler.)
2. Function handler also takes serverRequest as parameter.
3. These DSLs define handler.
4. Function handlers that these DSLs defined are now using mistakenly filterFunction's serverRequest, not their own serverRequest parameters, which makes their own parameters meaningless.

This bug only exists in Kotlin DSL.

You can see java codes have no problem as follows.

spring-framework/spring-webmvc/src/main/java/org/springframework/web/servlet/function/HandlerFilterFunction.java

Line 46 in 01e50fb

R filter(ServerRequest request, HandlerFunction<T> next) throws Exception;

spring-framework/spring-webflux/src/main/java/org/springframework/web/reactive/function/server/HandlerFilterFunction.java

Line 48 in 01e50fb

Mono<R> filter(ServerRequest request, HandlerFunction<T> next);

In java you can override filter something like

ServerRequest newRequest = someOperation(request)
return next.handle(newRequest)

but in Kotlin DSL, even if you write

filter { serverRequest, handlerFunction ->
    val newServerRequest = someOperation(serverRequest)
    handlerFunction(newServerRequest)
}

handlerFunction will use original serverRequest, not the newServerRequest

[sdeleuze]

Good catch, to be backported on 5.2.x as well. I will be able to merge the proposed fix with additional tests asked here, thanks.

[ShindongLee]

@sdeleuze I added tests. Thank you !