Fix to refresh the tip when it flips only (without offset change)

This bug happens if you show the qTip, where the Tip plugin is used and it has to do a flip, eg. options are set to:
```JavaScript
        position: {
            my: 'left center',
            at: 'right center',
            viewport: $(window),
            adjust: {
                method: 'flip shift',
```

Then qTip's Tips plugin has to do a flip, so my right center is at left center.  The Tip arrow remains facing the opposite way, because only corner.string() changes so it does not update().

I'll try and do a small example fiddle or enter a bug if you would like.