Copy headers map in RestClientResponseException to ensure serializability

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

Closes gh-31787

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

@@ -74,7 +74,7 @@ public class DefaultResponseErrorHandlerTests {
 		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