UnsupportedOperationException if modifyRequestBody uses LinkedMultiValueMap

  .route("user_route",r->r.path("/user")
                        .filters(
                                f->f.setRequestHeader("Content-Type", MediaType.APPLICATION_FORM_URLENCODED_VALUE)
                                .modifyRequestBody(MultiValueMap.class,MultiValueMap.class,((serverWebExchange, user) -> {
                                    MultiValueMap<String, String> formData = new LinkedMultiValueMap<>();
                                    formData.add("username", "John");
                                    formData.add("nickName", "qwerty");
                                    return  Mono.just(formData);
                                }))
                        )
                        .uri("lb://smweb"))

java.lang.UnsupportedOperationException: null
	at java.util.Collections$UnmodifiableMap.put(Collections.java:1457) ~[na:1.8.0_144]
	at org.springframework.http.HttpHeaders.set(HttpHeaders.java:1451) ~[spring-web-5.0.7.RELEASE.jar:5.0.7.RELEASE]
	at org.springframework.http.HttpHeaders.setContentType(HttpHeaders.java:854) ~[spring-web-5.0.7.RELEASE.jar:5.0.7.RELEASE]
	at org.springframework.http.codec.FormHttpMessageWriter.write(FormHttpMessageWriter.java:132) ~[spring-web-5.0.7.RELEASE.jar:5.0.7.RELEASE]
	at org.springframework.http.codec.multipart.MultipartHttpMessageWriter.lambda$write$1(MultipartHttpMessageWriter.java:188) ~[spring-web-5.0.7.RELEASE.jar:5.0.7.RELEASE]
	at reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:118) ~[reactor-core-3.1.8.RELEASE.jar:3.1.8.RELEASE]
	at reactor.core.publisher.Operators$MonoSubscriber.complete(Operators.java:1083) ~[reactor-core-3.1.8.RELEASE.jar:3.1.8.RELEASE]
	at reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:144) ~[reactor-core-3.1.8.RELEASE.jar:3.1.8.RELEASE]
	at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onNext(FluxOnErrorResume.java:73) ~[reactor-core-3.1.8.RELEASE.jar:3.1.8.RELEASE]
	at reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.onNext(FluxMapFuseable.java:115) ~[reactor-core-3.1.8.RELEASE.jar:3.1.8.RELEASE]
	at reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.onNext(FluxMapFuseable.java:115) ~[reactor-core-3.1.8.RELEASE.jar:3.1.8.RELEASE]
	at reactor.core.publisher.FluxFilterFuseable$FilterFuseableSubscriber.onNext(FluxFilterFuseable.java:104) ~[reactor-core-3.1.8.RELEASE.jar:3.1.8.RELEASE]
	at reactor.core.publisher.Operators$MonoSubscriber.complete(Operators.java:1083) ~[reactor-core-3.1.8.RELEASE.jar:3.1.8.RELEASE]
	at reactor.core.publisher.MonoCollectList$MonoBufferAllSubscriber.onComplete(MonoCollectList.java:117) ~[reactor-core-3.1.8.RELEASE.jar:3.1.8.RELEASE]
	at reactor.core.publisher.FluxMap$MapSubscriber.onComplete(FluxMap.java:130) ~[reactor-core-3.1.8.RELEASE.jar:3.1.8.RELEASE]
	at reactor.core.publisher.FluxPeek$PeekSubscriber.onComplete(FluxPeek.java:245) ~[reactor-core-3.1.8.RELEASE.jar:3.1.8.RELEASE]
	at reactor.core.publisher.FluxMap$MapSubscriber.onComplete(FluxMap.java:130) ~[reactor-core-3.1.8.RELEASE.jar:3.1.8.RELEASE]
	at reactor.ipc.netty.channel.FluxReceive.terminateReceiver(FluxReceive.java:377) ~[reactor-netty-0.7.8.RELEASE.jar:0.7.8.RELEASE]
	at reactor.ipc.netty.channel.FluxReceive.drainReceiver(FluxReceive.java:202) ~[reactor-netty-0.7.8.RELEASE.jar:0.7.8.RELEASE]
	at reactor.ipc.netty.channel.FluxReceive.request(FluxReceive.java:110) ~[reactor-netty-0.7.8.RELEASE.jar:0.7.8.RELEASE]
	at reactor.core.publisher.FluxMap$MapSubscriber.request(FluxMap.java:149) ~[reactor-core-3.1.8.RELEASE.jar:3.1.8.RELEASE]
	at reactor.core.publisher.FluxPeek$PeekSubscriber.request(FluxPeek.java:130) ~[reactor-core-3.1.8.RELEASE.jar:3.1.8.RELEASE]
	at reactor.core.publisher.FluxMap$MapSubscriber.request(FluxMap.java:149) ~[reactor-core-3.1.8.RELEASE.jar:3.1.8.RELEASE]
	at reactor.core.publisher.MonoCollectList$MonoBufferAllSubscriber.onSubscribe(MonoCollectList.java:90) ~[reactor-core-3.1.8.RELEASE.jar:3.1.8.RELEASE]
	at reactor.core.publisher.FluxMap$MapSubscriber.onSubscribe(FluxMap.java:86) ~[reactor-core-3.1.8.RELEASE.jar:3.1.8.RELEASE]
	at reactor.core.publisher.FluxPeek$PeekSubscriber.onSubscribe(FluxPeek.java:163) ~[reactor-core-3.1.8.RELEASE.jar:3.1.8.RELEASE]

Start at the modifyRequestBody route shown in the report and trace the request through FormHttpMessageWriter and MultipartHttpMessageWriter, the entry points named in the stack trace. Reproduce the LinkedMultiValueMap form request and determine the relevant gateway and Spring Web behavior; done means the route completes without UnsupportedOperationException and sends the form data successfully.