JSON.stringify(urlVar) should quote the string

My change at: https://codereview.qt-project.org/#/c/172431/4 improved the situation with URLs, but they are still broken. I didn't notice this when fixing the original issue because I was focused on getting output that I could print rather than making sure that the output was useful.

Item {
    property url source: "https://qt.io"
    Component.onCompleted: console.log(JSON.stringify(source))
} 

The above should produce "https://qt.io" (with quotes), so that it can be properly parsed at the other end.