Ensure WebClient exceptions are serializable

Not all HttpHeaders implementations are serializable. This commit
ensures that WebClientRequestException and WebClientResponseException
are serializable, by copying any non-serializable HttpHeaders into a
new, serializable, instance.

Closes gh-28321

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

@@ -17,6 +17,8 @@
 package org.springframework.web.reactive.function.client;
 
 import java.net.URI;
+import java.util.List;
+import java.util.Map;
 
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.HttpMethod;
@@ -56,7 +58,11 @@ public class WebClientRequestException extends WebClientException {
 	 */
 	private static HttpHeaders copy(HttpHeaders headers) {
 		HttpHeaders result = new HttpHeaders();
-		result.putAll(headers);
+		for (Map.Entry<String, List<String>> entry : headers.entrySet()) {
+			for (String value : entry.getValue()) {
+				result.add(entry.getKey(), value);
+			}
+		}
 		return result;
 	}
 

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

@@ -18,6 +18,8 @@ package org.springframework.web.reactive.function.client;
 
 import java.nio.charset.Charset;
 import java.nio.charset.StandardCharsets;
+import java.util.List;
+import java.util.Map;
 
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.HttpRequest;
@@ -47,7 +49,7 @@ public class WebClientResponseException extends WebClientException {
 	private final Charset responseCharset;
 
 	@Nullable
-	private final HttpRequest request;
+	private transient final HttpRequest request;
 
 
 	/**
@@ -112,7 +114,11 @@ public class WebClientResponseException extends WebClientException {
 		}
 		else {
 			HttpHeaders result = new HttpHeaders();
-			result.putAll(headers);
+			for (Map.Entry<String, List<String>> entry : headers.entrySet()) {
+				for (String value : entry.getValue()) {
+					result.add(entry.getKey(), value);
+				}
+			}
 			return result;
 		}
 	}