Allow LuaVal set metamethod to return value

This allows us to do something similar to the following:

local value = obj:Data():Get("key") or obj:Data():Set("key", val)

LuaValue.cpp

@@ -136,6 +136,7 @@ int LuaVal::lua_set(lua_State* L) {
     }
     else {
         (**p).insert_or_assign(std::move(kk), std::move(vv));
+        return it->second.asObject(L);
     }
     return 0;
 }