Merge branch '6.1.x'

Author: bclozel

spring-web/src/main/java/org/springframework/http/HttpHeaders.java

@@ -436,8 +436,11 @@ public HttpHeaders(MultiValueMap<String, String> headers) {
 		if (headers == EMPTY) {
 			this.headers = CollectionUtils.toMultiValueMap(new LinkedCaseInsensitiveMap<>(8, Locale.ENGLISH));
 		}
-		else if (headers instanceof ReadOnlyHttpHeaders readOnlyHttpHeaders) {
-			this.headers = readOnlyHttpHeaders.headers;
+		else if (headers instanceof HttpHeaders httpHeaders) {
+			while (httpHeaders.headers instanceof HttpHeaders wrapped) {
+				httpHeaders = wrapped;
+			}
+			this.headers = httpHeaders.headers;
 		}
 		else {
 			this.headers = headers;

spring-web/src/test/java/org/springframework/http/HttpHeadersTests.java

@@ -69,6 +69,14 @@ void constructorUnwrapsReadonly() {
 		assertThat(writable.getContentType()).isEqualTo(MediaType.TEXT_PLAIN);
 	}
 
+	@Test
+	void writableHttpHeadersUnwrapsMultiple() {
+		HttpHeaders originalExchangeHeaders = HttpHeaders.readOnlyHttpHeaders(new HttpHeaders());
+		HttpHeaders firewallHeaders = new HttpHeaders(originalExchangeHeaders);
+		HttpHeaders writeable = new HttpHeaders(firewallHeaders);
+		writeable.setContentType(MediaType.APPLICATION_JSON);
+	}
+
 	@Test
 	void getOrEmpty() {
 		String key = "FOO";