Change string copy policy: only string literal are stored by pointer

extras/tests/MsgPackSerializer/serializeVariant.cpp

@@ -139,7 +139,8 @@ TEST_CASE("serialize MsgPack value") {
 
   SECTION("str 32") {
     std::string shortest(65536, '?');
-    checkVariant(shortest.c_str(), "\xDB\x00\x01\x00\x00"_s + shortest);
+    checkVariant(JsonString(shortest.c_str(), true),  // force store by pointer
+                 "\xDB\x00\x01\x00\x00"_s + shortest);
   }
 
   SECTION("serialized(const char*)") {