Improved compatibility with older Solr schemas.

- Reduces conflicts with pre-improved-author-indexing designs.
- See discussion on pull request #650.

module/VuFind/src/VuFind/RecordDriver/SolrDefault.php

@@ -1056,8 +1056,8 @@ class SolrDefault extends AbstractBase
      */
     public function getPrimaryAuthors()
     {
-        return isset($this->fields['author']) ?
-            $this->fields['author'] : [];
+        return isset($this->fields['author'])
+            ? (array) $this->fields['author'] : [];
     }
 
     /**