Minor tweaks

Author: liuzicheng1987

include/rfl/internal/is_default_val_v.hpp

@@ -5,6 +5,7 @@
 
 #include "../DefaultVal.hpp"
 #include "../Rename.hpp"
+#include "StringLiteral.hpp"
 
 namespace rfl::internal {
 

tests/json/test_default_val.cpp

@@ -29,6 +29,7 @@ TEST(json, test_default_val) {
   EXPECT_EQ(homer.first_name, "Homer");
   EXPECT_EQ(homer.last_name.value(), "Simpson");
   EXPECT_EQ(homer.town.value(), "");
+  EXPECT_EQ(homer.home_country().value(), "USA");
 
   write_and_read(
       Person{"Waylon", "Smith", "Springfield"},