By adding a (*WriteFlusher).Close, we limit the Write calls to possibly
deallocated http response buffers to the lifetime of an http request.
Typically, this is seen as a very confusing panic, the cause is usually a
situation where an http.ResponseWriter is held after request completion. We
avoid the panic by disallowing further writes to the response writer after the
request is completed.
Signed-off-by: Stephen J Day <stephen.day@docker.com>
