Fix auth page content type

Fixes cesanta#172

Author: rojer

auth_server/server/server.go

@@ -349,7 +349,7 @@ func (as *AuthServer) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 
 // https://developers.google.com/identity/sign-in/web/server-side-flow
 func (as *AuthServer) doIndex(rw http.ResponseWriter, req *http.Request) {
-	rw.Header().Set("Content-Type", "text-html; charset=utf-8")
+	rw.Header().Set("Content-Type", "text/html; charset=utf-8")
 	fmt.Fprintf(rw, "<h1>%s</h1>\n", as.config.Token.Issuer)
 	if as.ga != nil {
 		fmt.Fprint(rw, `<p><a href="/google_auth">Login with Google account</a></p>`)