SEARCH-2202 Avoid returning mutable list.

search-services/alfresco-search/src/main/java/org/alfresco/solr/AlfrescoSolrDataModel.java

@@ -18,6 +18,8 @@
  */
 package org.alfresco.solr;
 
+import static java.util.Collections.unmodifiableList;
+
 import java.io.File;
 import java.io.FileInputStream;
 import java.io.IOException;
@@ -1339,7 +1341,7 @@ public class AlfrescoSolrDataModel implements QueryConstants
 
         public List<FieldInstance> getFields()
         {
-            return fields;
+            return unmodifiableList(fields);
         }
 
         public void addField(String prefix, boolean localised, boolean sort)