WebClient.mutate() should not impact future .mutate() invocations

Before this commit, when adding filters to a builder obtained via
`WebClient.mutate()`, the filters were added both to the original client
as well as the mutated builder. This commit fixes that.

Issue: SPR-16059

spring-webflux/src/main/java/org/springframework/web/reactive/function/client/DefaultWebClient.java

@@ -142,7 +142,7 @@ class DefaultWebClient implements WebClient {
 
 	@Override
 	public Builder mutate() {
-		return this.builder;
+		return new DefaultWebClientBuilder(this.builder);
 	}