Make registries return an object_t

All the registry helper functions should return an object_t, given that
they're guaranteed to return an object.  nlohmann::json as a type can
technically be string/int/bool/null/object/array, so it causes some
peculiarities in parsing.

Change-Id: If296477cb8d066d7f44ef0d12f17d94a5301e450
Signed-off-by: Ed Tanous <ed@tanous.net>
diff --git a/redfish-core/include/heartbeat_messages.hpp b/redfish-core/include/heartbeat_messages.hpp
index 61aadf0..8b7d3a5 100644
--- a/redfish-core/include/heartbeat_messages.hpp
+++ b/redfish-core/include/heartbeat_messages.hpp
@@ -27,7 +27,7 @@
 
 namespace messages
 {
-nlohmann::json redfishServiceFunctional();
+nlohmann::json::object_t redfishServiceFunctional();
 
 }
 } // namespace redfish