Fixing background image size when calling setFrame

If you init the segmented control with ```CGRectZero``` then later resize the control, the background will get stuck with a height of zero and not display.

eg:

```obj-c
- (id)init
{
    self = [super init];
    if (self) {
       _segmentedControl = [[AKSegmentedControl alloc] initWithFrame:CGRectZero];
    }
    return self;
}
```

...

```obj-c
   [self.segmentedControl setFrame:(CGRect){10, 0, self.view.frame.size.width-20, 37}];
```

Haven't seen any other issues introduced with this change but let me know if you think of something.