Fix incorrect weak ETag validation

Author: kashike

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

@@ -1073,8 +1073,8 @@ 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.startsWith("\"") || etag.startsWith("W/\""),
+					"Invalid ETag: does not start with W/\" or \"");
 			Assert.isTrue(etag.endsWith("\""), "Invalid ETag: does not end with \"");
 			set(ETAG, etag);
 		}

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

@@ -219,6 +219,12 @@ void illegalETag() {
 		assertThatIllegalArgumentException().isThrownBy(() -> headers.setETag(eTag));
 	}
 
+	@Test
+	void illegalETagWithoutQuoteAfterWSlash() {
+		String etag = "W/v2.6\"";
+		assertThatIllegalArgumentException().as("Invalid Weak ETag").isThrownBy(() -> headers.setETag(etag));
+	}
+
 	@Test
 	void ifMatch() {
 		String ifMatch = "\"v2.6\"";