Fix incorrect weak ETag assertion

Closes gh-33376

Author: kashike

Diff for: spring-web/src/main/java/org/springframework/http/HttpHeaders.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.
@@ -1044,9 +1044,8 @@ public long getDate() {
 	 */
 	public void setETag(@Nullable String etag) {
 		if (etag != null) {
-			Assert.isTrue(etag.startsWith("\"") || etag.startsWith("W/"),
-					"Invalid ETag: does not start with W/ or \"");
-			Assert.isTrue(etag.endsWith("\""), "Invalid ETag: does not end with \"");
+			Assert.isTrue(etag.startsWith("\"") || etag.startsWith("W/\""), "ETag does not start with W/\" or \"");
+			Assert.isTrue(etag.endsWith("\""), "ETag does not end with \"");
 			set(ETAG, etag);
 		}
 		else {

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

@@ -192,11 +192,17 @@ void ipv6Host() {
 	}
 
 	@Test
-	void illegalETag() {
+	void illegalETagWithoutQuotes() {
 		String eTag = "v2.6";
 		assertThatIllegalArgumentException().isThrownBy(() -> headers.setETag(eTag));
 	}
 
+	@Test
+	void illegalWeakETagWithoutLeadingQuote() {
+		String etag = "W/v2.6\"";
+		assertThatIllegalArgumentException().isThrownBy(() -> headers.setETag(etag));
+	}
+
 	@Test
 	void ifMatch() {
 		String ifMatch = "\"v2.6\"";