Make stream return headers immediately

Before that change, stream endpoint didn't return anything until either heartbeat or the first event happens. During that time, client wasn't been able say for sure whether connection has been established successfully or there is a problem with CORS or authorization.

Sending an empty message right after the connection has been established initiates the response sequence and triggers the webserver to respond with headers.
I assume this won't break message parsing on the client or anything like that?