Make sure the _uid field is correctly loaded in scripts.

Author: jtibshirani

server/src/main/java/org/elasticsearch/search/lookup/LeafFieldsLookup.java

@@ -147,14 +147,15 @@ private FieldLookup loadFieldData(String name) {
         }
         if (data.fields() == null) {
             String fieldName = data.fieldType().name();
+            String lookupField = fieldName;
             if (singleType && UidFieldMapper.NAME.equals(fieldName)) {
-                fieldName = IdFieldMapper.NAME;
+                lookupField = IdFieldMapper.NAME;
             }
-            fieldVisitor.reset(fieldName);
+            fieldVisitor.reset(lookupField);
             try {
                 reader.document(docId, fieldVisitor);
                 fieldVisitor.postProcess(mapperService);
-                List<Object> storedFields = fieldVisitor.fields().get(data.fieldType().name());
+                List<Object> storedFields = fieldVisitor.fields().get(fieldName);
                 data.fields(singletonMap(fieldName, storedFields));
             } catch (IOException e) {
                 throw new ElasticsearchParseException("failed to load field [{}]", e, name);