Guard against invalid response status

ServletWebRequest now guard against invalid HTTP response status when
processing conditional requests.

Issue: SPR-13516

spring-web/src/main/java/org/springframework/web/context/request/ServletWebRequest.java

@@ -244,11 +244,15 @@ public class ServletWebRequest extends ServletRequestAttributes implements Nativ
 
 
 	private boolean isCompatibleWithConditionalRequests(HttpServletResponse response) {
+		try {
 			if (response == null || !servlet3Present) {
 				// Can't check response.getStatus() - let's assume we're good
 				return true;
 			}
 			return HttpStatus.valueOf(response.getStatus()).is2xxSuccessful();
+		} catch (IllegalArgumentException e) {
+			return true;
+		}
 	}
 
 	private boolean isHeaderAbsent(HttpServletResponse response, String header) {

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

@@ -87,6 +87,18 @@ public class ServletWebRequestHttpMethodsTests {
 		assertNull(servletResponse.getHeader("Last-Modified"));
 	}
 
+	// SPR-13516
+	@Test
+	public void checkNotModifiedInvalidStatus() {
+		long epochTime = currentDate.getTime();
+		servletRequest.addHeader("If-Modified-Since", epochTime);
+		servletResponse.setStatus(0);
+
+		assertTrue(request.checkNotModified(epochTime));
+		assertEquals(304, servletResponse.getStatus());
+		assertEquals(dateFormat.format(epochTime), servletResponse.getHeader("Last-Modified"));
+	}
+
 	@Test
 	public void checkNotModifiedHeaderAlreadySet() {
 		long epochTime = currentDate.getTime();