func (s Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    r.ParseForm()
    log.Println(r.Form)
}

curl -d '{"foo":"bar"}' http://localhost:3000
prints: map[foo:[bar]]

curl -H "Content-Type: application/json" -d '{"foo":"bar"}' http://localhost:3000
prints: map[]