speedtest: fix mimetype on served JS

Change-Id: Ifcb1d4f8a58a5e6120f31373b2a8c0e307e414be
diff --git a/bgpwtf/speedtest/backend/main.go b/bgpwtf/speedtest/backend/main.go
index 7d4aece..3473352 100644
--- a/bgpwtf/speedtest/backend/main.go
+++ b/bgpwtf/speedtest/backend/main.go
@@ -134,9 +134,11 @@
 		w.Write(static.Data["bgpwtf/speedtest/index.html"])
 	})
 	http.HandleFunc("/speedtest.js", func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Add("Content-Type", "text/javascript")
 		w.Write(static.Data["bgpwtf/speedtest/speedtest.js"])
 	})
 	http.HandleFunc("/speedtest_worker.js", func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Add("Content-Type", "text/javascript")
 		w.Write(static.Data["bgpwtf/speedtest/speedtest_worker.js"])
 	})