Create case-insensitive copy in RestClientResponseException

Closes gh-31978

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2023 the original author or authors.
+ * Copyright 2002-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -19,6 +19,7 @@ package org.springframework.web.client;
 import java.io.UnsupportedEncodingException;
 import java.nio.charset.Charset;
 import java.nio.charset.StandardCharsets;
+import java.util.Locale;
 import java.util.function.Function;
 
 import org.springframework.core.ParameterizedTypeReference;
@@ -27,7 +28,8 @@ import org.springframework.http.HttpHeaders;
 import org.springframework.http.HttpStatusCode;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
-import org.springframework.util.LinkedMultiValueMap;
+import org.springframework.util.CollectionUtils;
+import org.springframework.util.LinkedCaseInsensitiveMap;
 import org.springframework.util.MultiValueMap;
 
 /**
@@ -102,7 +104,8 @@ public class RestClientResponseException extends RestClientException {
 	@Nullable
 	private static HttpHeaders copyHeaders(@Nullable HttpHeaders headers) {
 		if (headers != null) {
-			MultiValueMap<String, String> result = new LinkedMultiValueMap<>(headers.size());
+			MultiValueMap<String, String> result =
+					CollectionUtils.toMultiValueMap(new LinkedCaseInsensitiveMap<>(headers.size(), Locale.ENGLISH));
 			headers.forEach((name, values) -> values.forEach(value -> result.add(name, value)));
 			return HttpHeaders.readOnlyHttpHeaders(result);
 		}

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

@@ -0,0 +1,43 @@
+/*
+ * Copyright 2002-2024 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.web.client;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.http.HttpHeaders;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * @author Arjen Poutsma
+ */
+class RestClientResponseExceptionTests {
+
+	@Test // gh-31978
+	void caseInsensitiveHeaders() {
+		HttpHeaders headers = new HttpHeaders();
+		headers.set("foo", "bar");
+		RestClientResponseException ex = new RestClientResponseException("message", 200, "statusText", headers, null,
+				null);
+
+		HttpHeaders responseHeaders = ex.getResponseHeaders();
+		assertThat(responseHeaders).isNotNull();
+		assertThat(responseHeaders).hasSize(1);
+		assertThat(responseHeaders.getFirst("FOO")).isEqualTo("bar");
+	}
+
+}