Fix default end values for FlxRangeBounds and FlxPointRangeBounds

Currently if no value is provided for `endMin` and `endMax`, `endMax` will default to `endMin`, which in turn had defaulted to `startMin`.

This means that doing ```myRangeBounds.set(100, 200);``` would result in ```{start: {min: 100, max: 200}, end: {min: 100, max: 100}}```

Subsequently, the start and end values are not the same, and so the end values get used, meaning the final value of every particle is the minimum possible for any property.

This change adds an extra condition to fix that.