Fix showWithStatus not working after quick hide/show calls when isClear is true

This commit resolves the issue outlined and reproduced in https://github.com/samvermette/SVProgressHUD/issues/251

Essentially, whenever the HUD `isClear` evaluates to true, `-showProgress:status:maskType:` sets `self.hudView.alpha = 1`, but is blocked by the check `if(self.alpha != 1)`. The check should instead be `if(self.alpha != 1 || self.hudView.alpha != 1)`

[Line 608](https://github.com/samvermette/SVProgressHUD/blob/master/SVProgressHUD/SVProgressHUD.m#L608) shows a similar check being performed in `-dismiss`