Copy default headers, cookies in WebClient builder

This commit makes copies of the default headers and cookies when a
WebClient is built, so that subsequent changes to these do not affect
previously built clients.

Closes: gh-25992

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

@@ -274,9 +274,13 @@ final class DefaultWebClientBuilder implements WebClient.Builder {
 				.map(filter -> filter.apply(exchange))
 				.orElse(exchange) : exchange);
 
+		HttpHeaders defaultHeaders = copyDefaultHeaders();
+
+		MultiValueMap<String, String> defaultCookies = copyDefaultCookies();
+
 		return new DefaultWebClient(filteredExchange, initUriBuilderFactory(),
-				this.defaultHeaders != null ? HttpHeaders.readOnlyHttpHeaders(this.defaultHeaders) : null,
-				this.defaultCookies != null ? CollectionUtils.unmodifiableMultiValueMap(this.defaultCookies) : null,
+				defaultHeaders,
+				defaultCookies,
 				this.defaultRequest, new DefaultWebClientBuilder(this));
 	}
 
@@ -313,4 +317,28 @@ final class DefaultWebClientBuilder implements WebClient.Builder {
 		return factory;
 	}
 
+	@Nullable
+	private HttpHeaders copyDefaultHeaders() {
+		if (this.defaultHeaders != null) {
+			HttpHeaders copy = new HttpHeaders();
+			this.defaultHeaders.forEach((key, values) -> copy.put(key, new ArrayList<>(values)));
+			return HttpHeaders.readOnlyHttpHeaders(copy);
+		}
+		else {
+			return null;
+		}
+	}
+
+	@Nullable
+	private MultiValueMap<String, String> copyDefaultCookies() {
+		if (this.defaultCookies != null) {
+			MultiValueMap<String, String> copy = new LinkedMultiValueMap<>(this.defaultCookies.size());
+			this.defaultCookies.forEach((key, values) -> copy.put(key, new ArrayList<>(values)));
+			return CollectionUtils.unmodifiableMultiValueMap(copy);
+		}
+		else {
+			return null;
+		}
+	}
+
 }