This feature can be used in place of the pile of hacks in middleware.go,
and doesn't involve awkwardly shimming out a http.ResponseWriter. Sounds
like a win-win!

For sendfile(2) support. I should really DRY up the implementations here...

Package graceful provides graceful shutdown support for net/http servers,
net.Listeners and net.Conns. It does this through terrible, terrible hacks, but
"oh well!"