Quote stringified generic variants on JSON.stringify

A string representation of those is unlikely to be actual JSON, and even
if it is, we don't want to use it as such.

Fixes: QTBUG-72674
Change-Id: I6815366a0176d9725ff4840d3fc545792ce00535
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>
Reviewed-by: Robin Burchell <robin.burchell@crimson.no>

src/qml/jsruntime/qv4jsonobject.cpp

@@ -739,7 +739,7 @@ QString Stringify::Str(const QString &key, const Value &v)
     }
 
     if (const QV4::VariantObject *v = value->as<QV4::VariantObject>()) {
-        return v->d()->data().toString();
+        return quote(v->d()->data().toString());
     }
 
     o = value->asReturnedValue();

tests/auto/qml/qqmlecmascript/tst_qqmlecmascript.cpp

@@ -8464,7 +8464,8 @@ void tst_qqmlecmascript::stringify_qtbug_50592()
 
     QScopedPointer<QObject> obj(component.create());
     QVERIFY(obj != nullptr);
-    QCOMPARE(obj->property("source").toString(), QString::fromLatin1("http://example.org/some_nonexistant_image.png"));
+    QCOMPARE(obj->property("source").toString(),
+             QString::fromLatin1("\"http://example.org/some_nonexistant_image.png\""));
 }
 
 // Tests for the JS-only instanceof. Tests for the QML extensions for