fileserver: Fix Caddyfile parsing

Reported at https://github.com/mholt/caddy-sqlite-fs/issues/3

modules/caddyhttp/fileserver/caddyfile.go

@@ -78,7 +78,7 @@ func (fsrv *FileServer) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
 		return d.ArgErr()
 	}
 
-	for d.NextBlock(0) {
+	for nesting := d.Nesting(); d.NextBlock(nesting); {
 		switch d.Val() {
 		case "fs":
 			if !d.NextArg() {