Use the article domain as author for export files

When exporting an entry, use the domain name as author name for epub,
mobi and pdf formats, instead of 'wallabag'.
Change the author from array to string, because for now, there is always
only one author.

src/Wallabag/ApiBundle/Controller/EntryRestController.php

@@ -180,6 +180,7 @@ class EntryRestController extends WallabagRestController
         return $this->get('wallabag_core.helper.entries_export')
             ->setEntries($entry)
             ->updateTitle('entry')
+            ->updateAuthor('entry')
             ->exportAs($request->attributes->get('_format'));
     }