Fix SSE by implementing Flusher in responseWriter wrapper

[gfloyd]

I noticed after upgrading to any version later than 1.5.8 that server-sent events stopped working. It looks like the responseWriter wrapper added in #470 does not implement the http.Flusher interface, so events are never flushed to the client.

Adding Flush() to the wrapper fixes the issue and server-sent events are working for me again.

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[aaronhurt]

Thank you, nice catch!