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);
 	}
 
 

spring-webflux/src/test/java/org/springframework/web/reactive/function/client/DefaultWebClientTests.java

@@ -31,10 +31,8 @@ import reactor.test.StepVerifier;
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.MediaType;
 
-import static org.junit.Assert.assertEquals;
-import static org.mockito.Mockito.mock;
-import static org.mockito.Mockito.verifyNoMoreInteractions;
-import static org.mockito.Mockito.when;
+import static org.junit.Assert.*;
+import static org.mockito.Mockito.*;
 
 /**
  * Unit tests for {@link DefaultWebClient}.
@@ -136,17 +134,30 @@ public class DefaultWebClientTests {
 		builder.defaultHeader("foo", "bar");
 		builder.defaultCookie("foo", "bar");
 		WebClient client1 = builder.build();
+
 		builder.filter((request, next) -> next.exchange(request));
 		builder.defaultHeader("baz", "qux");
 		builder.defaultCookie("baz", "qux");
 		WebClient client2 = builder.build();
 
+		WebClient.Builder mutatedBuilder = client1.mutate();
+
+		mutatedBuilder.filter((request, next) -> next.exchange(request));
+		mutatedBuilder.defaultHeader("baz", "qux");
+		mutatedBuilder.defaultCookie("baz", "qux");
+		WebClient clientFromMutatedBuilder = mutatedBuilder.build();
+
 		client1.mutate().filters(filters -> assertEquals(1, filters.size()));
 		client1.mutate().defaultHeaders(headers -> assertEquals(1, headers.size()));
 		client1.mutate().defaultCookies(cookies -> assertEquals(1, cookies.size()));
+
 		client2.mutate().filters(filters -> assertEquals(2, filters.size()));
 		client2.mutate().defaultHeaders(headers -> assertEquals(2, headers.size()));
 		client2.mutate().defaultCookies(cookies -> assertEquals(2, cookies.size()));
+
+		clientFromMutatedBuilder.mutate().filters(filters -> assertEquals(2, filters.size()));
+		clientFromMutatedBuilder.mutate().defaultHeaders(headers -> assertEquals(2, headers.size()));
+		clientFromMutatedBuilder.mutate().defaultCookies(cookies -> assertEquals(2, cookies.size()));
 	}
 
 	@Test