JsonString: change default ownership to Copied

extras/tests/Misc/JsonString.cpp

@@ -96,7 +96,7 @@ TEST_CASE("JsonString") {
     JsonString s("hello world", 5);
 
     CHECK(s.size() == 5);
-    CHECK(s.isLinked() == true);
+    CHECK(s.isLinked() == false);
     CHECK(s == "hello");
     CHECK(s != "hello world");
   }

extras/tests/MsgPackSerializer/serializeVariant.cpp

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