Fix incorrect weak ETag assertion

Closes spring-projectsgh-33377

Author: kashike

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

@@ -783,12 +783,14 @@ public long getDate() {
 	 */
 	public void setETag(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);
 		}
-		set(ETAG, etag);
-	}
+        else {
+            remove(ETAG);
+        }
+    }
 
 	/**
 	 * Return the entity tag of the body, as specified by the {@code ETag} header.

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

@@ -157,6 +157,12 @@ public void illegalETag() {
 		assertEquals("Invalid ETag header", "\"v2.6\"", headers.getFirst("ETag"));
 	}
 
+    @Test(expected = IllegalArgumentException.class)
+    public void illegalWeakETagWithoutLeadingQuote() {
+        String etag = "W/v2.6\"";
+        headers.setETag(etag);
+    }
+
 	@Test
 	public void ifMatch() {
 		String ifMatch = "\"v2.6\"";