chg facet author to list

solr/gfl/conf/schema.xml

@@ -127,7 +127,7 @@
   <copyField source="id" dest="all_header_fields" />
   <copyField source="short_title" dest="all_header_fields" />
   <copyField source="title" dest="all_header_fields" multiValued="true"/>
-  <copyField source="author" dest="all_header_fields" />
+  <copyField source="author" dest="all_header_fields" multiValued="true"/>
   <copyField source="recipient" dest="all_header_fields" />
   <copyField source="origin_place" dest="all_header_fields" />
   <copyField source="destination_place" dest="all_header_fields" />