Fix zombie caused when resizing down orientation

Previous memory leak fix introduced a new bug when resizing image having UIImageOrientationDown.

Other orientations return an image created with: CGBitmapContextCreateImage()
Thus, the caller owns the ImageRef

Down orientation returns existing self CGImage unaltered, thus not owned by caller.
Merge pull request #2 from mikekloster/patch-2

Fix zombie caused when resizing down orientation