Search fix: fielddata_fields query string parameter was ignored.

The RestSearchAction did not parse the fielddata_fields parameter. Added test case and missing parser code.

Closes #11025

rest-api-spec/test/search/10_source_filtering.yaml

@@ -90,3 +90,9 @@
   - match: { hits.hits.0.fields: { include.field2 : [v2] }}
   - is_true:  hits.hits.0._source
   
+  
+  - do:
+      search:
+        fielddata_fields: ["count"]
+  - match: { hits.hits.0.fields.count: [1] }  
+

src/main/java/org/elasticsearch/rest/action/search/RestSearchAction.java

@@ -195,6 +195,20 @@ public class RestSearchAction extends BaseRestHandler {
                 }
             }
         }
+        String sFieldDataFields = request.param("fielddata_fields");
+        if (sFieldDataFields != null) {
+            if (searchSourceBuilder == null) {
+                searchSourceBuilder = new SearchSourceBuilder();
+            }
+            if (Strings.hasText(sFieldDataFields)) {
+                String[] sFields = Strings.splitStringByCommaToArray(sFieldDataFields);
+                if (sFields != null) {
+                    for (String field : sFields) {
+                        searchSourceBuilder.fieldDataField(field);
+                    }
+                }
+            }
+        }
         FetchSourceContext fetchSourceContext = FetchSourceContext.parseFromRestRequest(request);
         if (fetchSourceContext != null) {
             if (searchSourceBuilder == null) {