OutboundSseEvent is not correctly serialized

Closes #10673

This request allows special server sent events in the form of

```java
@GET
@Produces(MediaType.SERVER_SENT_EVENTS)
public Multi<OutboundSseEvent> consume(@Context Sse sse)

{ return Multi.createFrom() .item(sse.newEventBuilder().id(UUID.randomUUID().toString()) .mediaType(MediaType.TEXT_HTML_TYPE).name("whatever").data("<h3>EVENT DATA</h3>") .reconnectDelay(3000).build()); }

```
I'd like to write a test for that, but there is no existing one for the changed class. That's probably due to the static access of SseUtil.send() which is impossible to mock (as far as I know).