Fix issue with async return value type determination

Before this change, the type of asynchronously produced return values
(e.g. Callable, DeferredResult, ListenableFuture) could not be
properly determined with an actual resulting value of null. Or even
with an actual value returned, the generic type could not be properly
determined. This change fixes both of those issues.

Issue: SPR-12287

spring-web/src/main/java/org/springframework/web/method/HandlerMethod.java

@@ -247,7 +247,7 @@ public class HandlerMethod {
 	/**
 	 * A MethodParameter with HandlerMethod-specific behavior.
 	 */
-	private class HandlerMethodParameter extends MethodParameter {
+	protected class HandlerMethodParameter extends MethodParameter {
 
 		public HandlerMethodParameter(int index) {
 			super(HandlerMethod.this.bridgedMethod, index);