Support webflux.base-path in StripFilter and possibly others

Describe the bug
StripPrefix filter (and maybe RewritePath too) isn't able to remove a contextPath if there is any set. It started with upgrading to Spring Boot 2.3. I think it is connected with introducing spring.webflux.base-path but I don't use it. I use ForwardedHeaderTransformer for adding na prefix from a header X-Forwarded-Prefix (SCG runs behind NGINX proxy). This transformer sets contextPath to prefix builder.contextPath(prefix). StripPrefix filter strips only in path and not in contextPath request.mutate().path(newPath) and then it throws an IllegalArgumentException if build() method is called.
See a stacktrace:

java.lang.IllegalArgumentException: Invalid contextPath '/admin': must match the start of requestPath: '/sba/'
    at org.springframework.http.server.DefaultRequestPath.validateContextPath(DefaultRequestPath.java:82)
    Suppressed: reactor.core.publisher.FluxOnAssembly$OnAssemblyException:
 Error has been observed at the following site(s):
    |_ checkpoint ⇢ org.springframework.cloud.gateway.filter.WeightCalculatorWebFilter [DefaultWebFilterChain]
    |_ checkpoint ⇢ org.springframework.security.web.server.authorization.AuthorizationWebFilter [DefaultWebFilterChain]
    |_ checkpoint ⇢ org.springframework.security.web.server.authorization.ExceptionTranslationWebFilter [DefaultWebFilterChain]
    |_ checkpoint ⇢ org.springframework.security.web.server.authentication.logout.LogoutWebFilter [DefaultWebFilterChain]
    |_ checkpoint ⇢ org.springframework.security.web.server.savedrequest.ServerRequestCacheWebFilter [DefaultWebFilterChain]
    |_ checkpoint ⇢ org.springframework.security.web.server.context.SecurityContextServerWebExchangeWebFilter [DefaultWebFilterChain]
    |_ checkpoint ⇢ org.springframework.security.web.server.ui.LogoutPageGeneratingWebFilter [DefaultWebFilterChain]
    |_ checkpoint ⇢ org.springframework.security.web.server.ui.LoginPageGeneratingWebFilter [DefaultWebFilterChain]
    |_ checkpoint ⇢ org.springframework.security.oauth2.client.web.server.authentication.OAuth2LoginAuthenticationWebFilter [DefaultWebFilterChain]
    |_ checkpoint ⇢ org.springframework.security.oauth2.client.web.server.OAuth2AuthorizationRequestRedirectWebFilter [DefaultWebFilterChain]
    |_ checkpoint ⇢ org.springframework.security.web.server.context.ReactorContextWebFilter [DefaultWebFilterChain]
    |_ checkpoint ⇢ org.springframework.security.web.server.header.HttpHeaderWriterWebFilter [DefaultWebFilterChain]
    |_ checkpoint ⇢ org.springframework.security.config.web.server.ServerHttpSecurity$ServerWebExchangeReactorContextWebFilter [DefaultWebFilterChain]
    |_ checkpoint ⇢ org.springframework.security.web.server.WebFilterChainProxy [DefaultWebFilterChain]
    |_ checkpoint ⇢ org.springframework.cloud.sleuth.instrument.web.TraceWebFilter [DefaultWebFilterChain]
    |_ checkpoint ⇢ org.springframework.boot.actuate.metrics.web.reactive.server.MetricsWebFilter [DefaultWebFilterChain]
    |_ checkpoint ⇢ HTTP GET "/admin/sba/" [ExceptionHandlingWebHandler]
 Stack trace:
            at org.springframework.http.server.DefaultRequestPath.validateContextPath(DefaultRequestPath.java:82)
            at org.springframework.http.server.DefaultRequestPath.initContextPath(DefaultRequestPath.java:57)
            at org.springframework.http.server.DefaultRequestPath.<init>(DefaultRequestPath.java:42)
            at org.springframework.http.server.RequestPath.parse(RequestPath.java:60)
            at org.springframework.http.server.reactive.AbstractServerHttpRequest.<init>(AbstractServerHttpRequest.java:81)
            at org.springframework.http.server.reactive.DefaultServerHttpRequestBuilder$MutatedServerHttpRequest.<init>(DefaultServerHttpRequestBuilder.java:197)
            at org.springframework.http.server.reactive.DefaultServerHttpRequestBuilder.build(DefaultServerHttpRequestBuilder.java:136)
            at org.springframework.cloud.gateway.filter.factory.StripPrefixGatewayFilterFactory$1.filter(StripPrefixGatewayFilterFactory.java:71)
            at org.springframework.cloud.gateway.filter.OrderedGatewayFilter.filter(OrderedGatewayFilter.java:44)
            at org.springframework.cloud.gateway.handler.FilteringWebHandler$DefaultGatewayFilterChain.lambda$filter$0(FilteringWebHandler.java:118)

Sample
Configuration

spring:
  cloud:
    gateway:
      routes:
      - id: sba
        uri: http://localhost:20010
        predicates:
        - Path=/admin/sba/**
        filters:
        - StripPrefix=1

Enable ForwardedHeaderTransformer

@Configuration
public class ForwardedHeaderConfiguration {
  @Bean
  public ForwardedHeaderTransformer forwardedHeaderTransformer() {
    return new ForwardedHeaderTransformerImpl();
  }
}

Call http://localhost:{applicationPort}/sba with X-Forwarded-Prefix: /admin

Start with StripPrefixGatewayFilterFactory at the failing build path shown in the stack trace, then trace how ForwardedHeaderTransformer sets contextPath and how request mutation handles the path. Reproduce the sample route with spring.webflux.base-path or X-Forwarded-Prefix, and check whether RewritePath is affected too. Done means the request no longer throws when the context path is present and prefix stripping behaves correctly.