Polish DeferredResult

After this change a DeferredResult can be set before a
DeferredResultHandler has been set.

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

@@ -100,7 +100,7 @@ public class DeferredResultTests {
 		DeferredResult<String> result = new DeferredResult<String>();
 		assertFalse(result.isSetOrExpired());
 
-		result.getAndSetExpired();
+		result.expire();
 		assertTrue(result.isSetOrExpired());
 		assertFalse(result.setResult("hello"));
 	}