'retry:' isn't HTTP header. It's a field of SSE event. We should emit additional `\n` before 'retry:' to close HTTP headers section.

And I am confused about 'retry'. In accordance to SSE specs 'retry' and '\n\n' push new event with empty event type and empty data to a client. Such behaviour is undesirable. Do you see any practical worth in controlling 'retry' time? If you don't I prefer to remove this feature. I think it is almost useless.