Fix flat_settings REST parameter

* Get XContent params from request in Nodes rest actions
* Adding test for nodes info rest api

core/src/main/java/org/elasticsearch/rest/action/support/RestActions.java

@@ -275,7 +275,7 @@ public class RestActions {
 
         @Override
         public RestResponse buildResponse(NodesResponse response, XContentBuilder builder) throws Exception {
-            return RestActions.nodesResponse(builder, ToXContent.EMPTY_PARAMS, response);
+            return RestActions.nodesResponse(builder, channel.request(), response);
         }
 
     }

rest-api-spec/src/main/resources/rest-api-spec/test/nodes.info/30_settings.yaml

@@ -0,0 +1,19 @@
+---
+"node_info test flat_settings":
+  - do:
+      cluster.state: {}
+
+  - set: { master_node: master }
+
+  - do:
+      nodes.info:
+        metric: [ settings ]
+
+  - match : { nodes.$master.settings.client.type: node }
+
+  - do:
+      nodes.info:
+        metric: [ settings ]
+        flat_settings: true
+
+  - match : { nodes.$master.settings.client\.type: node }