Make testResponseWriter's CloseNotify return an actual channel instead of nil
Make testResponseWriter's CloseNotify return an actual channel instead of nil

This channel never gets written to, but this only means that the mock
ResponseWriter will never signal a premature disconnect.

Based on feedback from #763.

Signed-off-by: Aaron Lehmann <8ecfc6017a87905413dcd7d63696a2a4c351b604@docker.com>

docker/dctx