Core: fix upgrade response serialization

rest-api-spec/test/indices.upgrade/10_basic.yaml

@@ -0,0 +1,16 @@
+---
+"Basic test for upgrade indices":
+
+  - do:
+      indices.create:
+        index: test_index
+
+  - do:
+      cluster.health:
+        wait_for_status: yellow
+
+  - do:
+      indices.upgrade:
+        index: test_index
+
+  - match: {upgraded_indices.test_index: '/(\d\.)+\d/'}

src/main/java/org/elasticsearch/rest/action/admin/indices/upgrade/RestUpgradeAction.java

@@ -85,7 +85,7 @@ public class RestUpgradeAction extends BaseRestHandler {
             public RestResponse buildResponse(UpgradeResponse response, XContentBuilder builder) throws Exception {
                 builder.startObject();
                 buildBroadcastShardsHeader(builder, request, response);
-                builder.startArray("upgraded_indices");
+                builder.startObject("upgraded_indices");
                 for (Map.Entry<String, String> entry : response.versions().entrySet()) {
                     builder.field(entry.getKey(), entry.getValue(), XContentBuilder.FieldCaseConversion.NONE);
                 }