code: func SetContentType(contentType ContentType) func(next http.Handler) http.Handler {
  	return func(next http.Handler) http.Handler {
  		fn := func(w http.ResponseWriter, r *http.Request) {
  			r = r.WithContext(context.WithValue(r.Context(), ContentTypeCtxKey, contentType))
  			next.ServeHTTP(w, r)
  		}
  		return http.HandlerFunc(fn)
  	}
  }
docs: // SetContentType is a middleware that forces response Content-Type.