Dialog ignores content implicitWidth

import QtQuick 2.4
import QtQuick.Controls 1.3
import QtQuick.Dialogs 1.2

ApplicationWindow {
    visible: true
    width: 400
    height: 300

    Dialog {
        id: testDialog

        Rectangle {
            implicitWidth: 800
            implicitHeight: 400
            color: "lightblue"
            border {
                color: "red"
                width: 1
            }
        }
    }

    Button {
        anchors.centerIn: parent
        text: "Show dialog"
        onClicked: testDialog.open()
    }
}