[TIMOB-1647] Navigation Group on iPad does not destroy custom left & right nav buttons

Using 1.4 on 3.2 when a window sets a left & right nav button when that window is closed via the nav, those buttons are not destroyed from view. below code demonstrates.

// app.js //

var win1 = Ti.UI.createWindow({

backgroundColor: 'white',
title: 'Nav Group'

}); var label1 = Ti.UI.createLabel({

text: 'Open Child Window',
left: 10,
top: 10

}); win1.add(label1);

var group = Ti.UI.iPhone.createNavigationGroup({

window: win1

});

var main = Ti.UI.createWindow();
main.add(group);

main.open();

label1.addEventListener('click',function() {

var win2 = Ti.UI.createWindow({
    title: 'Child Window',
    backgroundColor: 'white'
}); 

var nextButton = Ti.UI.createButton({
    title: 'Next'
});

var backButton = Ti.UI.createButton({
    title: 'Back'
});

win2.rightNavButton = nextButton;

// win2.leftNavButton = backButton;

backButton.addEventListener('click',function() {
    group.close(win2);
});
group.open(win2);

});