tsweb: jsonhandler: fix content type

Signed-off-by: Zijie Lu <zijie@tailscale.com>

tsweb/jsonhandler.go

@@ -32,8 +32,8 @@ func responseError(e string) *response {
 
 func writeResponse(w http.ResponseWriter, s int, resp *response) {
 	b, _ := json.Marshal(resp)
-	w.WriteHeader(s)
 	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(s)
 	w.Write(b)
 }
 

tsweb/jsonhandler_test.go

@@ -44,6 +44,10 @@ func TestNewJSONHandler(t *testing.T) {
 			t.Fatalf("wrong status: %s %s", d.Status, status)
 		}
 
+		if w.Header().Get("Content-Type") != "application/json" {
+			t.Fatalf("wrong content type: %s", w.Header().Get("Content-Type"))
+		}
+
 		return d
 	}