Fixes interpretation of background=-1 while rotating an image.

https://github.com/joomla-framework/image/blob/master/Tests/ImageTest.php#L773 test is failing when executed locally.

`$image->handle` is false after rotation. I found out that when background=-1 is set for rotation then ` imagerotate($handle, $angle, $background);` returns false.

I assumed that if background=-1 is to make background transparent then we have an issue. This PR solves this issue.

<edit>Test fails only on php 5.x</edit>