Use sendError in ResponseStatusExceptionResolver

Prior to this commit, the `ResponseStatusExceptionResolver` would use:
* `HttpServletResponse.sendError` if both a status and a reason are set
on the `@ResponseStatus` annotation
* `HttpServletResponse.setStatus` if only a status is set on the
`@ResponseStatus` annotation

This is actually a change of behavior, since this Resolver was using
`sendError` in all cases previously.

Because this change can create issues such as
https://github.com/spring-projects/spring-boot/issues/3623
this commit rollbacks those changes and clarifies the behavior on the
javadoc of the annotation itself.

Issue: SPR-11193, SPR-13226

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/annotation/ResponseStatusExceptionResolver.java

@@ -100,7 +100,7 @@ public class ResponseStatusExceptionResolver extends AbstractHandlerExceptionRes
 			reason = this.messageSource.getMessage(reason, null, reason, LocaleContextHolder.getLocale());
 		}
 		if (!StringUtils.hasLength(reason)) {
-			response.setStatus(statusCode);
+			response.sendError(statusCode);
 		}
 		else {
 			response.sendError(statusCode, reason);