Specify where to write the MediaType of Sse event ?

We can set the Sse event media type with jaxrs api like:
```
OutboundSseEvent event = sse.newEventBuilder().name("json")
.data("{\"a\":\"b\",\"c\":\"d\")
.mediaType(MediaType.APPLICATION_JSON_TYPE).build();
```
But this setting isn't write and send to client like what http ContentType header does. Should spec define the behavior to write this information to stream? One possible place is event's comment field :
```
: application/json
event: json
data: {"a":"b","c":"d"}
```

Start with the issue's OutboundSseEvent example and the SSE stream format shown in the body. Determine whether the mediaType should be represented in the serialized event and where the specification should define that behavior; done means the specification has an agreed, interoperable rule.

Written by the indexing model from the issue text.