Support ETags with special chars in ServletWebRequest

This commit makes sure that HTTP request headers containing ETag values
are properly parsed and not simply tokenized using a "," separator.
Indeed, ETags can legally contain separator characters such as " " and
",".

Issue: SPR-14216

spring-web/src/test/java/org/springframework/web/context/request/ServletWebRequestHttpMethodsTests.java

@@ -144,6 +144,18 @@ public class ServletWebRequestHttpMethodsTests {
 		assertEquals(eTag, servletResponse.getHeader("ETag"));
 	}
 
+	@Test
+	public void checkNotModifiedETagWithSeparatorChars() {
+		String eTag = "\"Foo, Bar\"";
+		servletRequest.addHeader("If-None-Match", eTag);
+
+		assertTrue(request.checkNotModified(eTag));
+
+		assertEquals(304, servletResponse.getStatus());
+		assertEquals(eTag, servletResponse.getHeader("ETag"));
+	}
+
+
 	@Test
 	public void checkModifiedETag() {
 		String currentETag = "\"Foo\"";