Note: previously spaces weren't encoded, with this change they are (as plus) which is more conform to expected format for application/x-www-form-urlencoded
Actually this was a bug in previous version, as addQueryItem() didn't encode spaces as they should
See https://doc.qt.io/qt-5/qurlquery.html#addQueryItem
"This method does not treat spaces (ASCII 0x20) and plus ("+") signs as the same, like HTML forms do. If you need spaces to be represented as plus signs, use actual plus signs."
It wasn't an issue because none of passed data should contain spaces in our current code