Copy headers map in RestClientResponseException to ensure serializability

This commit ensures that the HttpHeaders used are serializable by making
 a copy.

Closes gh-31787

Author: poutsma

spring-web/src/main/java/org/springframework/web/client/RestClientResponseException.java

@@ -27,6 +27,8 @@
 import org.springframework.http.HttpStatusCode;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
+import org.springframework.util.LinkedMultiValueMap;
+import org.springframework.util.MultiValueMap;
 
 /**
  * Common base class for exceptions that contain actual HTTP response data.
@@ -88,11 +90,27 @@ public RestClientResponseException(
 		super(message);
 		this.statusCode = statusCode;
 		this.statusText = statusText;
-		this.responseHeaders = headers;
+		this.responseHeaders = copyHeaders(headers);
 		this.responseBody = (responseBody != null ? responseBody : new byte[0]);
 		this.responseCharset = (responseCharset != null ? responseCharset.name() : null);
 	}
 
+	/**
+	 * Copies the given headers, because the backing map might not be
+	 * serializable.
+	 */
+	@Nullable
+	private static HttpHeaders copyHeaders(@Nullable HttpHeaders headers) {
+		if (headers != null) {
+			MultiValueMap<String, String> result = new LinkedMultiValueMap<>(headers.size());
+			headers.forEach((name, values) -> values.forEach(value -> result.add(name, value)));
+			return HttpHeaders.readOnlyHttpHeaders(result);
+		}
+		else {
+			return null;
+		}
+	}
+
 
 	/**
 	 * Return the HTTP status code.

spring-web/src/test/java/org/springframework/web/client/DefaultResponseErrorHandlerTests.java

@@ -74,7 +74,7 @@ public void handleError() throws Exception {
 		assertThatExceptionOfType(HttpClientErrorException.class)
 				.isThrownBy(() -> handler.handleError(response))
 				.withMessage("404 Not Found: \"Hello World\"")
-				.satisfies(ex -> assertThat(ex.getResponseHeaders()).isSameAs(headers));
+				.satisfies(ex -> assertThat(ex.getResponseHeaders()).isEqualTo(headers));
 	}
 
 	@Test