Add true, false, and null to sample json

src/test.cpp

@@ -834,6 +834,9 @@ private:
 
 const std::string json = R"({
     "a number": 12345,
+    "true": true,
+    "false": false,
+    "null": null,
     "glossary": {
         "title": "example glossary",
 		"GlossDiv": {