fix lack of error on file site export

Previously, status 200 was set for file export, which is used
for backup, which resulted in an inability to set an error status code
in case of a problem with file generation.

After this change, status code 200 would be written automatically by Go
before we start writing the response's body.

backend/app/rest/api/migrator.go

@@ -136,7 +136,6 @@ func (m *Migrator) exportCtrl(w http.ResponseWriter, r *http.Request) {
 		exportFile := fmt.Sprintf("%s-%s.json.gz", siteID, time.Now().Format("20060102"))
 		w.Header().Set("Content-Type", "application/gzip")
 		w.Header().Set("Content-Disposition", "attachment;filename="+exportFile)
-		w.WriteHeader(http.StatusOK)
 		gzWriter := gzip.NewWriter(w)
 		defer func() {
 			if e := gzWriter.Close(); e != nil {

backend/app/rest/api/migrator_test.go

@@ -391,6 +391,15 @@ func TestMigrator_Export(t *testing.T) {
 	require.Equal(t, http.StatusAccepted, resp.StatusCode)
 	waitForMigrationCompletion(t, ts)
 
+	// export wrong site, should result in error
+	req, err = http.NewRequest("GET", ts.URL+"/api/v1/admin/export?mode=file&site=test", http.NoBody)
+	require.NoError(t, err)
+	req.SetBasicAuth("admin", "password")
+	resp, err = client.Do(req)
+	require.NoError(t, err)
+	require.Equal(t, http.StatusInternalServerError, resp.StatusCode)
+	require.Equal(t, "application/json", resp.Header.Get("Content-Type"))
+
 	// check file mode
 	req, err = http.NewRequest("GET", ts.URL+"/api/v1/admin/export?mode=file&site=remark42", http.NoBody)
 	require.NoError(t, err)

backend/app/rest/api/rss.go

@@ -56,7 +56,6 @@ func (s *rss) postCommentsCtrl(w http.ResponseWriter, r *http.Request) {
 
 	w.Header().Set("Content-Type", "application/xml; charset=utf-8")
 	w.WriteHeader(http.StatusOK)
-
 	if _, err = w.Write(data); err != nil {
 		log.Printf("[WARN] failed to send response to %s, %s", r.RemoteAddr, err)
 	}

backend/remark.rest

@@ -103,6 +103,14 @@ GET {{host}}/api/v1/admin/blocked?site={{site}}
 ### delete comment by id
 DELETE {{host}}/api/v1/admin/comment/3665976683?site={{site}}&url={{url}}
 
+### export site (for backup)
+GET {{host}}/api/v1/admin/export?site={{site}}&mode=stream
+X-JWT: eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJhdWQiOiJyZW1hcmsiLCJleHAiOjE5NzYwNTY3NTYsImp0aSI6IjJlOGJmMTE5OTI0MjQxMDRjYjFhZGRlODllMWYwNGFiMTg4YWZjMzQiLCJpYXQiOjE1NzYwNTY0NTYsImlzcyI6InJlbWFyazQyIiwidXNlciI6eyJuYW1lIjoiZGV2X3VzZXIiLCJpZCI6ImRldl91c2VyIiwicGljdHVyZSI6Imh0dHA6Ly8xMjcuMC4wLjE6ODA4MC9hcGkvdjEvYXZhdGFyL2NjZmEyYWJkMDE2Njc2MDViNGUxZmM0ZmNiOTFiMWUxYWYzMjMyNDAuaW1hZ2UiLCJhdHRycyI6eyJhZG1pbiI6dHJ1ZSwiYmxvY2tlZCI6ZmFsc2V9fX0.6Qt5s2enBMRC-Jmsua01yViVYI95Dx6BPBMaNjj36d4
+
+### export site (for backup) to .gz file
+GET {{host}}/api/v1/admin/export?site={{site}}&mode=file
+X-JWT: eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJhdWQiOiJyZW1hcmsiLCJleHAiOjE5NzYwNTY3NTYsImp0aSI6IjJlOGJmMTE5OTI0MjQxMDRjYjFhZGRlODllMWYwNGFiMTg4YWZjMzQiLCJpYXQiOjE1NzYwNTY0NTYsImlzcyI6InJlbWFyazQyIiwidXNlciI6eyJuYW1lIjoiZGV2X3VzZXIiLCJpZCI6ImRldl91c2VyIiwicGljdHVyZSI6Imh0dHA6Ly8xMjcuMC4wLjE6ODA4MC9hcGkvdjEvYXZhdGFyL2NjZmEyYWJkMDE2Njc2MDViNGUxZmM0ZmNiOTFiMWUxYWYzMjMyNDAuaW1hZ2UiLCJhdHRycyI6eyJhZG1pbiI6dHJ1ZSwiYmxvY2tlZCI6ZmFsc2V9fX0.6Qt5s2enBMRC-Jmsua01yViVYI95Dx6BPBMaNjj36d4
+
 ### get post info
 GET {{host}}/api/v1/info?site={{site}}&url={{url}}