ModifyResponseGatewayFilter is not using newContentType from Config

[muchnik]

Problem:
I wanted to modify response body with changing Content-type, but probably SCG ignoring newContentType
ModifyResponseGatewayFilter is not using newContentType in code as expected to set response 'Content-Type' header.

Don't see any usage of newContentType here:
https://github.com/spring-cloud/spring-cloud-gateway/blob/main/spring-cloud-gateway-server/src/main/java/org/springframework/cloud/gateway/filter/factory/rewrite/ModifyResponseBodyGatewayFilterFactory.java#L91

SCG version 3.1.2

Example:

    @Bean
    public RouteLocator customRouteLocator(RouteLocatorBuilder builder) {
        return builder.routes()
                      .route("r1", r ->
                              r.path("/megatest")
                               .filters(gatewayFilterSpec -> {
                                   return gatewayFilterSpec
                                           .modifyResponseBody(c -> c
                                                   .setInClass(String.class)
                                                   .setOutClass(String.class)
                                                   .setNewContentType(MediaType.TEXT_PLAIN_VALUE)
                                                   .setRewriteFunction((exchange, o) -> Mono.just(((String) o).toUpperCase())))
                                           .setPath("/target");
                               })
                               .uri("http://localhost:9999"))
                      .build();
    }

Current behaviour:
Response content-type is set from actual target response from external system.
I'am not getting 'text/plain' as expected

Expected behaviour:
Response content-type is set from newContentType field and equals 'text/plain'.

Further:
I can manage to fix this problem after discussion.

[spencergibb]

closing in favor of #2649