Revert "cmd/derper: stop setting content header in handleBootstrapDNS"

Didn't help enough. We are setting another header anyway. Restore it.

This reverts commit 60abeb027b.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

cmd/derper/bootstrap_dns.go

@@ -57,6 +57,7 @@ func refreshBootstrapDNS() {
 
 func handleBootstrapDNS(w http.ResponseWriter, r *http.Request) {
 	bootstrapDNSRequests.Add(1)
+	w.Header().Set("Content-Type", "application/json")
 	j, _ := dnsCache.Load().([]byte)
 	// Bootstrap DNS requests occur cross-regions,
 	// and are randomized per request,