Keep transparency when saving image on P mode

The problem I had was converting a .gif with transparent backgrount to
.png, it would turn the background black.

The first problem I found was on create_image() in case the animated gifs
are allowed. The convert() on each frame was changing the mode to RGB.

The second issue was just before save, the options it uses on Image.save()
didn't keep the 'transparency' present on Image.info.