EntriesExport: change authors and title when not single entry export

Change '{method} authors' (which gives 'Tag_entries authors' when
exporting a tag) to 'Various authors'.

When exporting a tag (tag_entries), change the title from 'Tag_entries
articles' to 'Tag {tag} articles'.

Signed-off-by: Kevin Decherf <kevin@kdecherf.com>

src/Wallabag/CoreBundle/Helper/EntriesExport.php

@@ -85,7 +85,7 @@ class EntriesExport
     public function updateAuthor($method)
     {
         if ('entry' !== $method) {
-            $this->author = $method . ' authors';
+            $this->author = 'Various authors';
 
             return $this;
         }