SSE clients in "connecting" state

[knopki]

The current SSE endpoint implementation (/stream) has a strange feature. There is no response at all until the first event. So, until the first event, clients stay in a "connecting" state. HTTP code is unknown, is connection alive is unknown.
It's very uncomfortable for client implementation, because you need to connect to the stream before making any other request, but you don't know are you connected or not. The workaround is to start ping-pong game, but it's not cool at all.

The fix is easy - just req.write(); req.flush(); after headers.
This is very connected to #57 onConnect callback can help with this.