Improved palette handling in ImageOps

[radarhere]

Three suggestions here for python-pillow#6596

1. You may have noticed that the tests are failing your pull request, with the error AttributeError: 'NoneType' object has no attribute 'copy'. This is because not all images have a palette. The palette doesn't actually need to be duplicated though, so I've removed the copy() operation.
2. I've added a test, to help prevent your change from being unintentionally broken in the future. After writing the test, I found that it was necessary to use getpalette(), and that ImageOps.expand() could also benefit from this change.
3. I notice that your code doesn't explicitly check the mode, allowing the palette data to be copied for both P and PA images. This is a good idea, and I've made that change for ImageOps.expand() as well.