adding tests

lib/Subscribers/ImportExport/Export/DefaultSubscribersGetter.php

@@ -44,7 +44,6 @@ class DefaultSubscribersGetter extends SubscribersGetter {
           SubscriberSegment::$_table . '.segment_id'
         )
       )
-      ->groupBy(Subscriber::$_table . '.id')
       ->groupBy(Segment::$_table . '.id');
 
     if($this->get_subscribers_without_segment !== false) {

lib/Subscribers/ImportExport/Export/DynamicSubscribersGetter.php

@@ -22,7 +22,7 @@ class DynamicSubscribersGetter extends SubscribersGetter {
     $segment_id = $this->segments_ids[$this->segment_index];
 
     $filters = Hooks::applyFilters(
-      'mailpoet_add_segment_filters', 
+      'mailpoet_get_segment_filters', 
       $segment_id
     );
     

lib/Subscribers/ImportExport/Export/SubscribersGetter.php

@@ -41,6 +41,7 @@ abstract class SubscribersGetter {
       )
     )
     ->filter('filterWithCustomFieldsForExport')
+    ->groupBy(Subscriber::$_table . '.id')
     ->whereNull(Subscriber::$_table . '.deleted_at');
   }