Fixed bug where using a custom image twice would fail

This code would not display the image when displaying the hud for the
second time:

```objc
    [MMProgressHUD
setPresentationStyle:MMProgressHUDPresentationStyleFade];
    [MMProgressHUD showWithTitle:@"AA" status:@"" image:[UIImage
imageNamed:@"1.png"]];
    [MMProgressHUD dismissAfterDelay:2.0];
    double delayInSeconds = 3.0;
    dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW,
(int64_t)(delayInSeconds * NSEC_PER_SEC));
    dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
        [MMProgressHUD showWithTitle:@"AA" status:@"" image:[UIImage
imageNamed:@"1.png"]];
        [MMProgressHUD dismissAfterDelay:1.5];
    });
```

Forcing an update takes care of the bug