ReadOnlyEndpoint: Adapt to apply return type change

Change-Id: I689fd2849be0c4af437500bea324878c753b68e8
diff --git a/src/main/java/com/googlesource/gerrit/plugins/readonly/ReadOnlyEndpoint.java b/src/main/java/com/googlesource/gerrit/plugins/readonly/ReadOnlyEndpoint.java
index 2482cab..a9e1c36 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/readonly/ReadOnlyEndpoint.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/readonly/ReadOnlyEndpoint.java
@@ -40,8 +40,8 @@
     }
 
     @Override
-    public String apply(ConfigResource resource) {
-      return state.isReadOnly() ? "on" : "off";
+    public Response apply(ConfigResource resource) {
+      return Response.ok(state.isReadOnly() ? "on" : "off");
     }
   }