CloseNotify() is deprecated

[perpetual-hydrofoil]

Unfortunately, CloseNotify() is deprecated:

"Deprecated: the CloseNotifier interface predates Go's context package. New code should use Request.Context instead." (https://golang.org/pkg/net/http/#CloseNotifier)

[perpetual-hydrofoil]

(Why this matters: it makes eventsource incompatible with most newer plugins/middleware that use context or that wrap the ResponseWriter like loggers, routers like fasthttp routers, etc.)