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
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